Step One: Asking her out.
Rainbow Dash is a very cool and nonchalant character, meaning that if you approach her with the intent of dating her, it should be done in a manner that does not seem too staged or contrived. In other words, 'just fly casual'.
We will be using a single character through this story in order to connect all of the dots and convey the right information, and it should be noted that while many of these approaches might work, it may not work for everyone. So let us cut back to him asking her out.
It had all started out as a casual conversation, with Twister Tail (Our 'you' allegory) explaining just why he couldn't go with her on a big adventure (A more common conversation with her than you might think). After you finish explaining this to her she asks you this question "So, what do we do about that then?"
He replies quickly, having thought about plans to see her before she had shipped off for her mission. "Well that depends on you really. I actually had a plan on taking you out for dinner or something, you know, for a date. Would you like to go on one with me?"
*Note: At this point, he has already learned many things about her personally. Always make sure to know who you're trying to go out with. This is rule 1 of dating.
She looks giddy for a brief moment (a sure sign that you have her hooked) but that's quickly replaced by a smug smirk. She clears her throat and says "Name the time and place bigshot."
He replies "7:00pm at Qin's Dynasty, nice fancy place without dress regulations."
*Note: Never ask Rainbow Dash to dress up for the first date. To some this may seem like retreading old ground, but since she is a tomboy by default, she dislikes most dresses and prefers to stay casual if she can help it.
*Additional Note: Inject humor into your conversation if you can, it eases tension and might show some of your wit.
He continues "No need to worry about dressing up there, just come as is with your money and your appetite. Oh wait, scratch that, come with my money and your appetite."
It hits the mark and she chuckles. "Alright then, sounds great. We eat at seven!"
Thus concludes step one.
Step Two: Dressing up.
While it has been stated that you shouldn't try to make Rainbow dress up, you should dress a little fancier yourself, because in most of these situations you will want to impress her. The only way this situation can be reversed is if /A/ You're a Wonderbolt, or /B/ You're such a big hero to her that she feels the need to impress you.
In this case, Twister is going through his clothes at his apartment trying to find something just right. Since he was the one that had brought up dress regulations, he needed to dress up at least a little bit for appearances.
He went through his two whole suits rather quickly as he had no intention of wearing them. He went through various other shirts and quickly eliminated them. They just weren't right for the situation at hand.
Eventually he decides to wear a simple black tie. After all, there's no need to be too fancy when you're eating Neighponese food. Besides that, assuming that this is just dinner as opposed to a more formal Equestrian holiday (take your pick) you only need the bits to buy food and maybe a few other things, should you need it.
Step Three: The Dinner.
When going to dinner, it's always important to arrive at the place first so that your date is not waiting upon you. This is a general bit of dating advice and can be applied to most women with few exceptions. He arrives five minutes before seven.
*Note: You might want to go earlier; some women may try to test you by going up to fifteen minutes early.
Four minutes later, his date arrives on the scene, exactly the same as you'd usually see her, minus an obvious attempt to brush her hair. He tries flirting in this instance.
"Really Mrs. Dash, you didn't have to doll yourself up for my sake." He speaks this remark with a hint of superiority.
She shoots back "Yeah, well you didn't have to put on that cheesy tie, but you did anyway." (*Note: Never try and act superior to Rainbow Dash, she is a bit sensitive about being anything less than the best.)
He replies "Touché. Shall we head inside for dinner then?" She agrees and they head into the restaurant.
If you are wondering what it was they decided to eat that night, it was what we would call Chinese. Beyond that, I leave it to your imagination. The point is, they were having dinner and enjoying time spent together.
Conversation can be quite tricky to those who do not know what to say, or more importantly what not to say to her. To know this, we must learn of her personality so that we may talk to her without insulting or belittling her. For example, bragging to her about how much better you are then her is a sure fire way to turn her into your rival.
On the other hand (or hoof, depending on your preference), you do not want to act like a total suck up to her either. All this will do is tell her that you are a 'yes man' and will blindly repeat whatever she thinks and says.
The trick is to find a healthy median between the two, by showing her that you care about her, but can also take care of yourself. As a side note, having an adventure with her is a good way of doing this without speaking to her.
A little bit of background for our protagonist, Mr. Twister Tail. He is a royal guard and a part of the night watch, so you could say that he has a lot of courage. Despite that fact however, the butterflies he's getting in his stomach right now are making him stutter worse than a guard trying to deliver a message to the princesses while they're having a bath. (Don't think that can't happen. It has. And it was awkward for most of the parties involved.)
In this instance, he's trying to tell her that she either looks good or smells nice. The only problems are /A/ he can't decide which one to say, and /B/ he's feeling like he's in a dream, not sure if he really did ask her out or not. (Ah love, how you torment our minds with thoughts of romance.)
She is in the middle of eating what's on her plate, and he feels that the right time to speak is soon at hand. 'But what if I say the wrong thing? What if I can't find the right words? What if I don't say anything at all?' he wonders. He acts on the last thought and take s a moment to breath before blurting out this sentence.
"Your hair smells looks good nic-OH WHY DID YOU SAY THAT?!"
He then grabs his head and slams it onto the table. The way he sees it, he had one chance and he blew it. Now he's gonna have to hear about it and he's never gonna live it down.
Rainbow Dash had fortunately been preoccupied and didn't notice. "Huh, what wuzzat?" she asks.
He sighs in relief and says "Well, I was just saying that you look wonderful tonight."
Her muzzle scrunches up a bit. "What do you mean tonight? I look good all the time."
"Well yea, but you look particularly beautiful with your hair all done up. You smell nice too."
She blushes a bit. "Jerk. Don't think that complimenting me is gonna get you anywhere, I don't fall for stuff like that easily." She looks quite happy about it, he thinks to himself.
He replies "Sure, you keep telling yourself that, I won't stop you."
A round of good natured laughing followed after that, along with some dialogue that, for all intents and purposes, has no bearing on this particular story. But what is the point of this amusing anecdote? Simple. Just because you fail the first time doesn't mean it's the only chance you get. Just try again. *Warning, you might get a few restraining orders if you try this for everyone you meet.*
Step Four: Afterwards
First thing to do is pay the bill. The second thing is to figure out what to do next. Some people will make a big grand plan about it, others will just go with the flow, and even more will ask their date what they want to do.
Twister chose option three. Dash replied that she didn't know and didn't really care all that much, and so he decided to take her to Canterlot's night markets. These parts of the town are where the late shoppers go to get supplies, or the more salacious or brave go to have a good time. Many stalls that wouldn't be able to show their wares in Celestia's sun proudly display themselves in the light of Luna's moon.
Many sights, smells, and sounds permeate through this bizarre bazaar, including the traipsing of a piano, the strumming of a guitar, a dash of rose and the flashing of lights, along with chilling masks from Zebrica, perfumes from Saddle Arabia, and the sounds of crowds in the streets. (Fun fact, the night markets are holed up in the 'worst part' of Canterlot, an area you might call the slums.) Twister knew the area quite well, so he knew where to take his date without running into the more… salacious parts of town.
They were currently in what might be considered the square, right near the middle where the statue of a prancing pony stands tall. Covered stalls surround the square, and there seems to be some sort of dance going around the middle statue, and a violin player giving the music they were dancing to.
Thinking quickly, Twister offered her a hand and asked "Care for a dance?"
She responded "Hmm, I dunno dude, violins aren't exactly my style. Besides, what's the point in dancing anyway, can't be anymore romantic and date worthy than anything else right?"
He retorted "Don't knock it till you try it. As a matter of fact, I'll give you one dance, complete satisfaction or your money back guaranteed."
Dash snorted "You're on pal! Give it your best shot."
Twister bowed. "As you wish." He then walked over towards the violin player and started talking with her, asking what songs she knew and what she was willing to play. When they were finished talking they shook hands, and he gave her a few bits for the request, then he walked back over to Dash and pulled her into the square.