There's no set rules or foundations needed when making an avatar tbh, just make sure it's the proper size of 150x200 or 150x300 and it would help if you had a proper program such as photoshop or gimp to use for making avatars. There's this small
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you could use, but avatars don't really need too much to be good. Usually just some colour effects and/or lighting effects would suffice. Along with a border of course :3.