When in Barnett College, go down into the hallway with all the rooms off of it. The first room on the right side is the courtyard. Walk through it to get to the art room. In the art room you will see 2 customizable characters. When you walk up to them, the customization process will start automatically.
Press the left and right to scroll through the customization types, and press up and down to scroll through the options. One of the options will be a chalkboard with "ABC" written on it. Use this option to change the name of the character, press to enter the text input mode. Change one option of the character, and go and change the name. Once you change the name, press and the change will automatically save and the achievement will unlock.
You can also use this customization options to create a wrench/shovel/etc. character to use that ability in free play without having to purchase that character from the library or unlock them through story completion.