This is a way to make heads with flat caps. You start with a sprue from Warlord Games' Anglo-Zulu War British Infantry:
Start by drilling a hole and putting the head on a paper clip or similar, so it's easier to work with. Then you cut off the top of the helmet:
Then you should cut off the back of the helmet, so that the bottom edge is in line with the brim. You should try to form the back of the head so it looks like hair.
Now you just have to form the rounded top of the hat, I do this with a combination of a file and scraping with a knife. The end result should look something like this:
Some painted flat caps made this way can be seen here: