As long as you learn how to use any image editor like Photoshop, Krita, Gimp or even Pixlr, it'll be super easy. :) Just make a wide transparent area, put the shape you want, colour it and make sure to save it as a PNG without losing the transparent area.

I don't think I could make a valuable tutorial on this since it's so easy. :P I think everything that I could say was already said by @ddua.