When I try to do an ace, I sometimes do a fault. I can sometimes do it, but is there a consistent way of doing an ace?

Personally, the way I do it is i swing my arm wildly all the way from behind my body and arcing it over my head to the front of my body, and I very rarely fault (although it isn't 100%). The motion is nothing like a real tennis serve, and it probably looks incredibly stupid, but it works for me.