I use a fence and do mine real similar to MGBurch. Except the forearm on my stock is close to finish dimension. I find the center of each lug and transfer that to the barrel with a pencil. Then I place the barrel back in the stock and transfer that line to the stock. This gives me the center of the lug in the X axis, I established the distance from the barrel previously after clamping the barrel to the fence and setting the tool tip the distance away from the barrel I want it. This distance will remain constant through the entire process.
To drill I chuck a small center drill up then move the stock and barrel to center on my pencil mark and clamp both against the fence. Very slowly, because I'm drilling into a slope, I bring the center drill down until I get a good spot watching all the time for signs of it running off center. Then I chuck a drill bit one number size under my pin size and all the way through both stock and lug using the method Taylor described. Then I chuck up the correct drill size and drill that all the way through.
Be extra careful when drilling through the metal tab as that is when bit is most likely to grab and break.
