You have to start with the minor matter of whether you are dealing with a plastic (hollow) NC or a balsa (solid) one.
For balsa, drill a hole as deeply as you dare from the base toward the nose or tip. Don't go too close to the tip or your added ballast could break through sometime in the future (like if a parachute snaps open at high speed, or your rocket lands hard, or you get a lawn-dart). Anyway, if you need only a little weight then use modeling clay or anything you have handy. If you need a lot of trim weight you may have to step up to copper shot (buy a pkg of BB ammo at your hardware store) or lead shot (buy a box of shotgun shells, pry open the front of a cartridge, pour out the shot, and dispose of the rest SAFELY). Pour some epoxy into the hole you drilled, pour in the shot, cover with some more epoxy.
Side note: Making a hole in a NC (as described above) also allows you to add a more secure point for attaching a screw eye. Drill your hole with a bit that matches whatever dowel rod size you can find at your local hardware store, like 1/4 inch, or 5/16, or 1/2. Fill the front of the hole with ballast, and fill the back of the hole with a plug cut from the dowel rod. This gives you a hard piece of wood into which you can securely insert your eye screw.
For a plastic NC things are usually a bit easier. Many of these NCs already have a small hole in the base as a result of the molding/manufacturing process. If not, cut or drill a small hole. Pour in some epoxy and shot (as above) and cover with a bit more epoxy. If you want to be double-darn sure that the ballast will not come loose, fill the remaining empty space with some foam-in-place product.
Return of the side note: If you add ballast to the NC be sure to also add a length of elastic shock-absorbing cord, so the weight of the NC does not cause it to snap the tether. Ballasted free-flight objects descending over the launch area on an uncontrolled ballistic path are generally discouraged.
Night of the living side note: If you are worried about an excessively aft center of gravity, you can help yourself a bit by keeping the recovery system as far forward as possible. Especially if you have a very long BT, the parachute and wadding will tend to slide aft under the force of the motor thrust. You can hold these items forward by placing your ejection baffle (or any other retaining device) about six or eight inches inside the front of the BT to keep things forward. Granted, the parachute usually doesn't weigh much, but it is better to keep it in the nose than let it slide toward the tail.
