Pottermore nickname Question?

[David]

Just start calling yourself whatever you want as your nickname. Like, if you want to be called (just as an example) Peachy Orangutang, start signing your comments with it (i.e. Argh! Where's the chocolate frog card! I can't find it! - Peachy Orangutang). Put it into your status (which you can update by going to your profile) or just make a general announcement in your common room (i.e. Welcome new members! If you need help, just ask. - Peachy Orangutang).

Really, it's just up to you to make it up and get others to call you it. There's nothing special that you have to do like signing up for a nickname.