It depends on the shaft size. For the standard small engine with a 1/4" shaft you can use a 1/4" drill bit. I have done it by just using my hand to grip the drill bit and turning enought to just enlarge the hole without getting it too large so that the prop does not center itself on the shaft. You want the hole too large so that you can get a wobble when the prop spins. You will also want to balance the prop after reaming it.