There's no really a single way to do them. I don't know how you usually paint your helmets, so I really can't say what's the easiest way for you.
If you're familiar with paths and vector layers, this is the easiest way, as vector data allow you a direct control on the shape not directly linked the pixels. So, make a vector layer with the shape of the flames and then add some shading to get those nice effects they have.
Otherwise, you can get there by using polyline selections. You make a selection with the shape of the flame and then fill it. It's a bit more difficult, as correcting the selection when you're not satisfied of it's shape is more tricky that with vector data. But if you're not familiar with this one, it could be better.
I started doing flams by selections, until I discovered how to use paths and vector layers. Just choose the one you feel more confortable with...