Well, first of all, I’m not; I just have fun doing it, and second of all it’s going to be the same answer no matter who you ask- I drew them a lot. I draw them a lot. I will continue to draw them a lot and I will never be as good at drawing them as I want to be, but I will be as good at drawing them as I can be, and that’s really all anyone can ask of you, so that’s fine :D
As for tips, apart from drawing them a lot and drawing as many from life as you can, try looking at the shapes of and in people’s faces. You can learn a lot from looking at skulls and the muscles of the face, but try picking out the repeating motifs in people’s faces- are there circles? Steps? Angles? Curves? What can you make that say about the face, about the person? Can you use that in a way that misleads the audience? Is your villain made of triangles and spikes, or are they a cutesy baby-faced joker?
Just have fun with it and draw as much as you can, whenever you can, of whatever you can- or, even better, what you can’t.