Snoke was one of those Sith wannabees that Palpatine trained to do his dirty work but didn't want to them to be actual Sith (the Rule of Two and all that). You know, Snoke was trained like those fellows with the propeller lightsabers in "Rebels," and Naare from "The Freemaker Adventures," if anybody considers that canon.
Unlike those guys, Snoke was much less ambitious and basically acted as an errand boy for Palpatine, up to and including trimming the Emperor's toenails and giving him a sponge bath.
After the Empire fell, Snoke escaped notice and got a desk job deep down in Corusant. He did OK but he was not much of a people person, and had truly terrible dating skills, so he was lonely and horny a lot. He got a sex robot to make up for the lack of living company.
One day, after some bitter disappointments trying to pick up strange at a bar, Snoke came home to discover that the power was out. He really wanted to be cajoled and told that everything would be OK by his sex robot but it was completely out of charge. Without thinking about it too much, Snoke used "electric fingers" to power up the robot.
The next morning, he looked in the mirror and saw that his face was dry and withered, just like old Sidious. Ah well. He told people at work that it was a rare medical condition that suddenly struck people in his family.
A few weeks later, Snoke got trashed and came home, fumbling in the dark. He couldn't find the light switch and stumbled down a hallway. Some light came in through a window and he saw an intruder. He grabbed for his blaster and shot at the figure. The blast bounced off the mirror at the end of the hallway that was reflecting his physiognomy and hit him on the head, leaving that big old scar that we see in the movies.
After coming back from the hospital, Snoke decided it was time he did something productive with his life, like building up a new empire, or he was going to end up freaking killing himself by accident!