This is my own guide that I recently made for this achievement.
Head down into the Helicarrier Command Center and through the door on the left. Go all the way down this hallway and through the door at the other side. Now stay right until you make it to the Medical Bay. In here, jump in one of the pods and all you have to do is change the name to create your character!
If you choose to name your created character 'Trevor' you will also unlock:Solution for The Toast of Croydon in LEGO Marvel Super Heroes
Watch the video as I show you exactly how to do this. If you have any questions, please leave a comment and let me know if there is anything I can do to improve my solution, thanks!