These are the steps I followed for making an adorable leather-covered mini top-hat!
Firstly, I cut a loo roll* down to size for the tube part (I know, so 'Blue Peter') and cut a circle out of stiff card. Out of this I cut the centre circle for the top of the hat, using a sharp cutting knife. This circle has to be equal to the internal size of the tube, even a few millimetres smaller depending on the thickness of the fabric covering it.
(*If making a full-size hat, this would of course be too small, and the circumference of the doll's head would have to be used as the correct circumference)
Then I cut a piece of leather long enough to cover the circumference of the loo roll tube plus an extra 1-2cms to make a neat fold-over at the join. I also gave it 1cm extra width at each side, so that it could be folded over and glued to the inside of the tube. Again for this I used UHU glue because it dries fast and glues leather rather well (although you usually have to keep the tube upright so it doesn't squish everywhere).
Then I cut two equal circles out of leather with which to cover the brim of the hat, with a smaller circle out of the middle.
One side of the card was covered in glue - I used UHU glue, because I've found this to bind leather very well especially on the soft, fuzzy inner side and it dries quickly, but I'm sure PVA would do the trick too if used carefully. The card was placed centred on one of the leather circles and once secure, I cut small triangle darts into the leather just short of the card, using sharp little embroidery scissors. This was so that there wouldn't be lots of bunched up leather where the curves cause it to fold. I then spread glue on the card bit by bit and stuck down the leather to it, working around the outside of the brim first. I then repeated this on the inside, in the hole of the brim, glueing this inner part on top of the secured outer rim.