Basically, in the comments you enter what Pokémon you are in your Steam name. Even if someone else is already the Pokémon that you want to be, you can still be that, but use a varied name to differentiate yourself. It helps if you use a unique icon. A short bio is helpful, but not necessary.
For instance, based on my name I'm a Mewtwo that is based on Kazuhira Miller, cloned from Mew by Big Boss.
And that's it.