In my opinion, If you're serious, you need one. But if you just started, you don't have to have one yet. Get one after having like, maybe more than 'at least' 50 subscriber/favorite because believe me, lots of people quit because they don't get subscriber or likes.
'how to do one/choose one' is easy. Just imagine it in your head. Every utaite/youtaite has their image in their head (with exceptions maybe?). But if you want to visualize them, then draw them. If you can't draw, you can ask someone to draw for you. Who will draw for you? your friends maybe? or pay someone to do so. Sometimes, some artist are kind enough to draw for free. Another way is trading. Example : you can mix, so you mix for someone and that someone will draw for you.
I think I sound a little harsh? No? If yes, I apologize.