The method geh3 has suggested below is good but I will add a couple of points.
1 - Look up a "Vanessa" balance rig - easy to make and they work on pretty much every plane.
2 - don't forget to subtract the weight of the epoxy you are going to use when determining the lead shot to be used.
ORIGINAL: geh3
I use #12 lead shot quite frequently.
First I determine how much is needed after I have mounted all of the
internal components as far forward as possible (so as to minimize
adding dead weigh to any airplane). Then I hang the plane at the recommended CG point
using a small simple ''block and tackle'' rig with 2 downlines, one on each side of the plane ( this only works for planes
with wings that slide onto wing tubes!). Once the plane is hoisted up a few inches, I add various weights
by taping then to the nose OR by taping a small paper cup to the nose and adding the lead shot to that until
the plane hangs ''level'' . Then I mix some 30 min epoxy and add in the premeasured lead shot. This mix is poured into the
nose cone up front, then the fuse is turned up on it's nose and the epoxy is allowed to cure. I recheck the CG of course
after the epoxy cures..