Despite how I prime the model, I start by painting the burning area white. The left side is how things should look when we finish.
Next, I do a heavy drybrush of yellow. You should be covering up the vast majority of the white (even in the crevices). Some white should show through, but not very much. I use a medium brush here.
After the yellow, I do a slightly lighter drybrush of orange. This is going to be the color that shows the most. Once again you want to cover the majority of the yellow. Some yellow should show through (especially around the areas where you can still see white. This stage also benefits from a medium brush.
Once we switch to red, we are no longer going to be drybrushing. You want to use a smaller brush, and you just want to highlight the individual wisps of flames. This is going to depend greatly on your model. With these dogs, I was painting (what was supposed to be) fur. Your model may have actual flame, so things may be a bit different. Highlight the tips of the flames and work down, but don't make it all the way to the base.
For the last bit of highlighting, we want to use a darker red. This should be used sparingly, but we want to highlight the tips of the areas we just painted red.
Now you want to mix some Bloodletter Glaze with some Lahmian Medium. The more medium in the mixture, the greater the contrast there will be on your final result. These photos don't properly show how much contrast there is in the flames. I used a 50:50 mix for these models here.
Finally I took the time to clean up the black areas.
I hope this has been helpful, painting flames is something that can be quite intimidating. This process can also be used for other colors as long as the core ideas of highlighting with darker colors are followed. I have sparingly used glazes in the past, but flames seem to be a great use for them.
I am out for the next few weeks while my wife and are galavanting around Europe. When we get back I will finish up the hounds.