Select the trait you want to change. When you click it, it should turn red, and you can use the arrow keys to change it. Click the next trait and repeat. Then, if you're done, click next. Now you can click a "change" button, and type in what you want it to say. Repeat. When you finish, click "done" and you can use your computer's screenshot button to save your new kitty (if you don't know where this is, look it up. It's different on each computer model).
NOTE: Please complete each feature. For example, if under clan you type in "Thunder", your clan will be "Thunder", not "ThunderClan", unlike other creations. All of the art is made by FunWarriorCats, I did none of it (well, I added a few mouths and changed the chest patches). In the original, everything is generated for you, and it's more complicated. See the link to the original game down below. (I recommend it!)