Here's how I freehand flames on my Salamanders. (Note: the colors are all flexible and you can use whatever you have at home.)

1. Start by sketching the basic shape of the flame you want in pure Army Painter Burning Ore . (This may take a few layers.)
2. Then go over it again with a 1:1 ratio of Burning Ore and Vallejo Flat Yellow , but leave the edges orange.
3. Go over again with pure Flat Yellow , once again leaving the edges showing.
4. Do the same thing with a 1:1 ratio of Flat Yellow and Pro Acryl Bold Titanium White .
5. Then again with pure Bold Titanium White .
6. (Optional) If you're painting over an armor panel with a groove in it, thin down some Abaddon Black and drip it into the cracks.
7. (Optional) I like to sponge over the flames with the background color, in this case Abaddon Black , and, if it's a vehicle, some silver (I use Gunmetal Grey ) for chipping.