The foreend is usually pretty easy. Use a caliper to measure the full width, then divide by 2 and use the caliper again, marking the midpoint.
On the buttstock, there can be more of a problem simply because stocks are finished by hand and they are very often not even. If you measure and put the stud in where the measurement says, it may appear to be off to the side. So what I usually did was to eyeball it, making sure the mark "looked right" no matter where it should have been by measurement. Brownells has a jig that is just a right angle piece of iron that you can put on the stock, but I don't think you will need it. (You can make one using cutout cardboard.)
BTW, you really need a special tool for that job. It has an end cutter through which the drill passes; the drill does the hole, and the end cutter cuts the flat or slight countersink so the stud sits flat rather than looking like it is balanced on a fence. Center punch and use a drill press, too; stocks are hard and it is very easy to let the drill wander when using a hand drill.
Needless to say, remove the barrelled action from the stock before drilling. For foreends, you will want to use a swivel with a machine screw and a ferrule inside in the barrel channel. It makes a better setup. (Swivel sets usually come that way, but there are some that just use wood screws for both swivels.)
Jim