tmix
If you don't wan't to buy a kit from the store, you can always take the head off put it down on top of some cardboard, trace a circle from a can or bowl etc. and cut it out with a razor knife.(you may want to smooth the inside of hole with some sandpaper where it is sharp).
Just remember, the smaller the hole the more secondary tone (longer sustain) you will get from your head the larger the hole the more it will sound like a single headed bass. (generally short quick decay).
Also, I've had better luck putting in the hole offcenter not disturbing the center of the skin so I get more tone and less whooshing air.
tmix