Draw all you want! Riot loves seeing community ideas. I believe thats how Riot Singed and Uncle Ryze started.

The reason most custom skins are computer made is that they are actually usable by modding the game files. They are only visible to you though.

I think you guys are missing it. I think hes talking about drawing a skin. To do this you need to print out the mesh outline, then trace your drawing over the mesh outline. Remember things outside the outline will not be on the model. Then you need to scan your work, and resize it to fit the scale of the skin image. You'll most likely want to touch it up in photoshop still though. I've seen skins in other games done this way before, my only tip is to make sure you fully color the image "sketch" art doesn't work. That or slap a photoshop filter on it afterwards. Hope this helps