Does anyone know how to make a custom skin?

I was wondering, I saw a video where someone changed the colour of project ashe (ill link it), and I wanted to know where can I make a custom league of legends skin, i've been wanting a new warwick skin for a while, and I know riot wont listen to me alone. So can anyone link me to where I could do that, Many thanks in advance for the help :) Link to the video where he changed the colours: https://www.youtube.com/watch?v=Bd803d5yez0