Third Person POV:
One would think Shirou's magecraft training would be much easier after he unlocked his real circuits, but it was not that simple. Although it was way less painful and he had a lot more magical energy, he still wasn't seeing much success when trying to cast spells. Even the most basic Reinforcement spell was too hard and he kept making everything he cast it on explode. He tried to ask Kiritsugu for advice, but he was told that Reinforcement was the easiest spell and that he just didn't have any talent as a magus. None of the other spells Kiritsugu taught worked either, but Shirou never once thought it was Kiritsugu's fault. Maybe he really didn't have any talent for magecraft, but that wouldn't stop him and he diligently kept up his daily practice anyway.
One day, he was trying the reinforcement spell as usual, when he somehow managed to create a copy of the metal pipe he was holding. This was the first time any kind of magecraft he tried worked, and he immediately ran to Kiritsugu to celebrate. Unfortunately, his excitement would be short-lived after he learned what he did was called Projection. As he watched Kiritsugu easily break the copy as if it was made of glass, he learned that Projection is a useless spell that wastes a lot of energy to create temporary and hollow replicas. Disappointed, he went back to practicing Reinforcement.
About three weeks after Shirou started to learn magecraft, Kiritsugu started packing up to go overseas. He had gone multiple times before during the last two years, and often the trips would last months at a time. By now, Shirou was already used to taking care of the house and his own needs himself, seeing as Taiga was more hindrance than help whenever she came over to "look after him". After he sent off Kiritsugu, Shirou returned home to get back to his usual schedule. On the way home however, he felt the urge to go to the convenience store and buy a pack of plastic knives. It went smoothly as the cashier didn't really care if a child played around with cheap knives that had a hard time even cutting cold butter, and Shirou was soon back in his shed with the bag.
He didn't know why he suddenly had the urge to suddenly buy these things, but he felt they would be very useful for his magecraft practice. He started the preliminary step of Reinforcement, which was called Structural Grasp and immediately felt a difference. The spell worked much better than usual and the information he received was much clearer as well. Excited, Shirou proceeded to pour magical energy into the knife and… the knife exploded. Unfortunately, Reinforcement wasn't that easy to master. Shirou took out another of the remaining 19 knives and tried again. After 3 more failed tries, the fifth knife didn't explode, but he still didn't cast the spell properly and ended up making the knife less effective and hollow. It wasn't until the twelfth time that Shirou finally felt that something was different.
He swung the reinforced knife at one of his metal pipes, and watched in awe as the plastic knife actually managed to scratch it. He had finally succeeded in Reinforcement for the first time. He wanted to keep trying, but he was already out of magical energy. Even if he wasn't, he couldn't just practice magecraft all the time as he still needed to do the chores and go to school. Speaking of school, ever since the magecraft training began, he has had a sudden increase in interest when it comes to studying science and math. Kiritsugu was way happier with this than the magecraft training and bought a shelf full of textbooks to try and support him. With Kiritsugu's tutoring and the textbooks, he was quickly getting through the curriculum for the third year of Elementary school and was soon going to learn things above his level.
With his sudden increase in grades, he drew a bit of attention from his classmates. Unfortunately, Shirou wasn't interested in making new friends or hanging out with other people. Whenever he thought about doing anything fun, he would be reminded of the fire and all the people he abandoned to live. Why should he be allowed to be happy when he stole that opportunity from all of them? Other than helping out the teacher with a few tasks and tutoring those who asked for help, he did not spend much time in school after class.
After coming home, Shirou began practicing kendo in the dojo. Previously, he only liked kendo when he got to train and spar with Kiritsugu. He used to stay away from the dojo when Kiritsugu wasn't around to escape Taiga, but now he felt an urge to practice more just like with studying. Starting with the warm-ups and stretches, then moving on to the most basic swings, thrusts, blocks and steps. He practiced over and over again, for a full three hours just like all the other days. Seeing his sudden diligence, Taga had even offered to teach him some advanced techniques. If it had been before, Shirou would have been very excited. But now, he rejected her because he had a feeling that learning the fundamentals and getting his body used to sword fighting was way more important than learning fancy techniques.
Homework was next on the list of daily activities after kendo. Shirou also used this time to read through the textbooks and study. Without Kiritsugu around, he had a lot more things he didn't understand and a lot more questions that went unanswered; but while Shirou was never an academic genius, he made up for it with perseverance. After doing the chores, cooking dinner, and eating with Taiga, he went to sleep so that he could wake up early to practice magecraft. More of the 16 remaining knives were broken in the morning, but there were also three more successful attempts at Reinforcement. When his magical energy ran out again, he headed to school after cooking breakfast and lunch. This daily schedule continued during the weekdays and he would visit Taiga's family during the weekends.
With the aid of the plastic knives, Shirou's Reinforcement was making rapid progress in regards to success rate and effectiveness. However, strange changes started happening when he performed the first step of Structural Grasp. He started to get information on how the knives were mass produced in the factory, the places they were shipped around to, and the time they spent on the shelves in the store. The even weirder part was that sometimes, this information appeared in his head when he just looked at the knives instead of touching them. He didn't know what this meant, but something told him it was a good thing. He tried to replicate these effects with other objects, but it was unsuccessful. The only thing he could do was keep trying.
After about a month of training with the plastic knives, only 3 remained intact and the other 17 were destroyed. But their sacrifice wasn't in vain, as Shirou could now cast Reinforcement with a 100% success rate on them and it was so effective that the reinforced knives could cleanly cut through the metal pipes. With his new achievements, Shirou excitedly went back to trying to cast Reinforcement on other objects, but the results were unsatisfactory. It was as if practicing with the plastic knives was like riding a bike with training wheels, and now the training wheels were gone. This was not only because he had a strange affinity with them, but because the knives were simple objects that were basically just 1 piece of plastic that was shaped a certain way.
Other objects had multiple components, complex structures and were made of all kinds of different materials. Fortunately, the previous training did give him the knowledge of how to reinforce something properly and increased his skill with Structural Grasp and magical energy control. Due to this, he was much more adept at the spell than he was a month ago and was improving at a much faster rate as well. After another 2 months of training, he was able to reinforce rocks, sticks, and pieces of paper to be harder than steel. As for the reinforced metal pipes, they were so tough that Shirou bet that they would be able to deflect bullets from even the most powerful guns that he had seen at Raiga Fujimura's place without a scratch. The success rate was only about 80-90% but it was improving rapidly and he was sure it would reach 100% soon. Once that was done, he would move on from the simple objects and try with the more complex ones.
Kiritsugu returned home about 3 months after he left. Just like the previous times, he seemed both tired and depressed. Shirou wanted to ask him about it, but he always avoided the question and didn't want to talk about it. After settling in, Kiritsugu asked about what happened during his trip and Shirou's progress with his magecraft. He was quite surprised when Shirou showed him how much better he had become with only 4 months of practice. Although Reinforcement is considered the foundation of all magecraft that enhance a target, it is also the most difficult of such magecraft. It is similar to how using one's bare hands is the most primitive method of digging a hole in the ground, but it is also far more difficult when compared to building and using various tools to do it instead.
Initially, Kiritsugu specifically chose to poorly teach Shirou this spell because he believed it would be far too difficult for him to figure out. He assumed that after months or years of making no progress, Shirou would give up learning magecraft and just live as a regular person. If Shirou asked to learn other spells, he could use Reinforcement as an example of his lack of talent and perseverance to shut him up. But now, all those plans have gone down the drain. The only thing he could do now was ramp up his reminders and warnings to Shirou regarding using magecraft for good and not allowing normal people to know of its existence. He might even have to consider teaching Shirou about hypnosis and bounded fields in order to hide himself better.
The hypnosis wouldn't be anything too advanced, just the most basic type that only works on normal people. He used to rely on it a lot when he had to evacuate civilians from dangerous locations where he hunted other magi. As for bounded fields, just a few simple ones will do. A warning type bounded field like the one around their house, as well as a sound-proof type, camouflage-type, and one that can hide small magical energy fluctuations should be enough. Although he was against Shirou becoming a magus, it was imperative that he teach him how to hide the existence of magecraft from regular people now that he knows it. Exposing the knowledge of magecraft to the public was the fastest way to anger the mage's association and have them dispatch their enforcers.
Life wasn't too different for Shirou after Kiritsugu returned, except for one thing. The strange new ideas he used to get about how to improve his magecraft stopped appearing. Instead, he felt the urge to spend more time studying and practicing kendo. Of course, that didn't stop him from continuing to practice magecraft, and he was making steady improvements all the time. One thing he was having trouble with however was remembering all the information he was getting from structural grasp as well as all the things he was studying. Kiritsugu decided to help solve this problem for the sake of his studies and taught him about one of the functions of magic circuits. They could record and store information similarly to a computer and could even be used as a calculator.
This was how magic crests were made possible, as all the research a magus did could be stored in the magic circuits and passed on. Shirou learned that most modern magi have magic crests that they inherited from their parents, which would give them an incredible advantage over a first generation magus like himself. Kiritsugu also had a magic crest, but he wasn't a blood-related member of the Emiya family so it was incompatible with him. At most, Shirou could sell the remaining part of the crest for money or favors from other magi.