What is the most logical next event?

How to colorize a drawing in gimp
Open gimp and browse to the folder that you have your drawing at.
If you don't have one, just do a search for drawings. It's a good idea to make sure that the license is public domain or creative commons.
If you are just beginning at this, go for a large image and clean lines. 1000 x 1000 is a good size to start with.