Here's what I did:
1. I made a pair of templates by tracing around cylindrical things (a mug and a small perfume bottle) onto some scrap cardboard (a chocolate packet). I cut it out, and repeated for the second one.
2. I wound a little ball of yarn, small enough to fit through the holes in the templates.
3. Holding the two templates sandwiched together, I wound the yarn around them until I decided I'd covered them pretty well. If you run out of yarn, it's easy to add another little ball - just wind the new yarn over the loose ends to keep them under control.
4. Get the point of your scissors down between the two cardboard templates, and carefully snip through the yarn all around the edge. I used fuzzy, woolly yarn which stayed put during the snipping - if your yarn is less 'grabby', I'd suggest plugging the centre of the circle with a folded paper towel or something, to stop the snipped strands of yarn escaping.
5. Wrap a new strand of yarn around the middle of the pompom, between the two templates. Tie it tightly with a secure knot.
6. Slip off the cardboard templates, trim any longer strands that are sticking out, and fluff up your pompom: roll it in your hands, or hold it by the tail and whack against your hand/leg/whatever.
7 (optional). To make the tail more substantial, take a crochet hook and with both strands, make a slip knot as close to the pompom as possible. Chain 4, and tie off.
8. Make more pompoms as desired, and attach to your hat!