Most of my terrain is hand made from pretty basic stuff, the fires are no exception.
They're actually made of tin-foil, I made a bunch of them a while back to be used for stuff like this.
There is two important things to follow, the first is scrunch the tinfoil up really tight and then tease it out again, that gives loads of texture to the foil, try and shape it a little organically from there, fire flows quite a lot, these 2 areas of flame were made for some large braziers I use in my pulp set ups. The second trick it to paint backwards, white, light yellow, proper yellow, orange, red, deep red, black or something similar, goes against the natural grain of painting and highlighting but fires are hottest in the middle and echoing this makes the tin foil shape really feel like fire.
I seal my tinfoil with PVA and gave them a white undercoat after that.
G