Practice practice practice!!
Since you are specifically interesting in working for Riot as well, you can look at all the posts some of the Reds have made where they describe in detail the processes they go through while designing champions and skins.
They just came out for one for the Star Guardian line of skins and they periodically have been releasing them for champions as well.
Study the people who you admire!
This is what I've been telling my friend who is in thesis photography right now. She's a bit lost on the whole thesis part, so I've been telling her to find some photographers she really admires, study their work, but even more importantly, read their words! They're able to do it as a career so you want to follow in their path in terms of what they do, but always put your own spice & twist to it.
Good luck on your artistic journey!
P.S: To whoever down voted this kids post, fuck you. He/she will probably go farther than your negative ass.