I got one of my santa hats and measured around it. I folded the purple faux fur in half because I only wanted one seam (to go at the back of my head).
After cutting the triangle out I unfolded it and cut a length of (sparkly) white fur to go along the bottom.
Next I sewed up the purple fur from the bottom to the point.
The tricky part comes next, getting the white fur onto the hat the right way. I put the white fur right side into the hat and sewed it to the wrong side, so when it's down the fur is pointing inwards. Then I sewed the fur to make it a circle and folded it upwards. Lastly I stitched the white fur up in 2 places to hold it up and have it the right way.
And finally when that was done I took a scrap of the white fur, loosely tacked around the edge and pulled it into a ball with a bit of stuffing and sewed it onto the point.