Bassman:
Hats! The everlasting question :hat .
I found it easier than I thought.
Start with the simplest one, like the fire sorcerer in my blog (link at the bottom), get a bunch of GreenStuff, roll it on a smooth surface, water it a lot, so it does not stick to the surface, use more blue than green (so it is harder).
To avoid finger prints I used a round pencil (water it, spit it!) to roll the GS bunch.
Shape it like a cylinder or, if you feel confident, like a cone.
Do not care about the two ends of the cylider, it’s the side that should be regolar…
This is important, as it is almost impossibile to have regular side and flat bases!
At least, I never manage to get them… 
Let it harden… in the meanwhile roll some others…
When they are hardened get a knife (x-knife, hobby tool, whatever you want) and CUT the bases!
If you have done it carefully you have a nice GS hat, with the dimensions, the shape you like…
I try to shape it as much circular as I can but with arrows, decoration and painting nobody will notice the inevitable little irregularities.
Here are mine!
https://discourse.chaos-dwarfs.com/t/8827