When I've noted in the past that the rear trunnion (underfold) itself makes an excellent template, there are some other items I thought I should mention:
1) There is a special tool used for cutting off rivet heads before you drill out the original rivets (this only applicable if you have a used receiver with fixed trunnion). Boeing Surplus has 'em for a buck. Forget what they're called. You can use the replacement rivets you have to pick a drill-out bit size. On that note, if there's a hole from the original rivets on a fixed-to-underfold conversion, the stock assy itself will conceal them.
2) When lining up the holes for drilling, there's a delicate dance involving getting the angles right for the drill bit - I used to do one side by eye and marking, sometimes leaving the trunnion inside the receiver and watching where the drill bit passed. I would use an undersized drill bit for this purpose so as not to drill out the trunnion itself. The holes don't have to be perfect, just close <g> as the rivets squish out into the holes.
3) During this process (hole drilling and alignment) you can put bolts thru the holes you have already made so that you have a fixed reference point (eg the trunnion doesn't move around as you drill/rivet).
4) Underfold stock hole: the dremel alum. oxide grinding stone (looks like a christmas tree or pear) is great for dressing this hole; the sanding drum is just about the size of the hole in the trunnion itself. I worked with these until I could just slip the sanding drum inside the receiver hole - smoothing it out. Perfect fit.
5) Locking pins. This assembly (which you will want to have functional if the AWB sunsets - methods of legal compliance are up to you, I was using a pre-ban donor) consists of two nuts (for lack of a better word) with two studs per nut, each projecting to the right. They lock into the holes in receiver/trunnion to fix the stock, and are each linked by pins on the "axle" of the stock - the threaded tube that the spring fits inside. If you have one of these you'll know what I'm talking about. The "locking pins/nuts" themselves have a little step/engagement area which as-is was just deep enough to engage the receiver holes only and did not fit into the trunnion. I had no guidance on this and used the dremel (again) to extend them so that they would lock solidly into the trunnion itself (eg pass thru receiver and engage deeper). This seemed a better idea to me, but I don't know if it was correct.
6) (per DrugRunr) you will need to make a small hole on the underside of the receiver, covered by the pistol grip when assembled - this is so you can knock out the "third pin" that appears to control spring tension/depth it sits in the tube of the stock. I neglected to do this and have to push pretty hard/far to get the stock to disengage (also see "5" above!) but will rectify this later and install).