First I start off by painting the base color of Averland Sunset (Citadel). Not pictured, but the same base color is used on the sunforge hammer, of course.
Then I did a layer of Flash Gitz Yellow (Citadel) over the base.
Next I made a wash from White Scar (Citadel) by mixing it with a bunch of water. You are pretty much looking to make milk with it. The stuff should wash on and fill in all the recesses and show the yellow through.
Then drybrush Flash Gitz Yellow (Citadel) heavily over it, keeping the white in the recesses. You can allow the following drybrushing colors to end up on nearby parts because they help create a glow effect.
Using Fire Orange (Reaper) I drybrushed over the hammer head, but lighter so as to not cover the white and yellow. On the wings I started partway down and drybrushed to the ends.
A similar step, I used Phoenix Red (Reaper) and did a lighter drybrushing, to create a gradient showing the material cooling.
To finish the effect I very carefully and lightly drybrushed Abaddon Black (Citadel) to show the coolest areas of the hammer and wings. If I felt an area needed more black, I was still very careful to lightly drybrush to build up to the level I thought looked good. It is very easy to do too much black and screw the whole thing up, so be careful.
And that's it! The finished product should look like a piece of iron heated to a white hot in the furnace, and slowly cooling, which of course starts on the outermost edges and works its way to the core.