How do you download custom skins?? :O

Don't do it.

Chance it is fine, but equal chance that somehow riot will be having a bad day, or a system will pick it up as some sort of cheat or exploit by accident, and they have the right to ban you.

Just not worth the risk anymore.

Xonra's comment is definitely valid (since doing this breaks the EULA, they're not responsible for fixing your game if it breaks, and you might have to reinstall), the free software Wooxy (http://mapskins.com/wooxy/) does it for PCs. A Mac version is in the works, but no exact date on when that'll come out.