I want to create a custom map

I mean, artistically you can do whatever you want. Drawing or 3d modelling is totally fine as long as it doesn't interact with the game directly.

If you mean a custom map to play on, that's not acceptable as it's against the rules (not even sure if it's possible, but still.) Even custom map skins are against the rules as it can give players an unfair advantage. In general, custom map and champion skins are off limits.