You say detent pins, I take it you mean the take-down and pivot pin detents?
The take-down pin is no big deal, but let me give you a tip for the pivot pin install. Go down to Home Depot and buy a piece of 1/4" aluminum rod stock (like a dowel). The holes in the lower for the pivot pin are nominally 1/4" but I found that you'll need to sand the rod a tad so it will move easily in the pin lugs. After you sand it finish with a little steel wool so it will be nice and smooth. Find a drill bit just a tad bigger than the detent and drill a hole through the rod maybe 1/8" or a little more from the end. Stick this through the takedown pin lugs from the opposite side, feed the spring and detent through the hole you drilled in the rod and push it in with a small allen wrench or whatever. Then, ease the rod back so the hole moves off the detent. Now, all you have to do is put your pivot pin up against the rod and feed it through, and the detent will pop into the channel in the pivot pin. This tool costs a couple of bucks and will add years to your lifespan, believe me. Or you can spend fifteen bucks and order the tool from Brownells, etc.
The other thing I'd tell you is take a file and blunt the tip of the detent. Otherwise it will really dig into the little stops in the pivot/takedown pins and make those a bit hard to shift when you want to open the weapon or remove the upper. If you ever need to remove the pivot pin, a scratch awl works well to hold the detent down so you can move the pin out - but put a rag or something over the lower or have a spare spring and detent ready 'cause those suckers will launch for lunar orbit if you let them loose.
The detent for the takedown pin is really easy. Leave it for the very end, when you put the stock on. Screw in the buffer tube, situate the buffer tube detent (speaking of lunar orbit, don't forget about that detent if you ever have to remove the buffer tube!) and slide the stock on. Feed the detent and spring in then install the buttstock screws. No brainer.
Selector detent: flip the lower over, drop the detent and spring in the hole. Go ahead and drop the screw into the pistol grip, then use your screwdriver or allen wrench to hold it in place. Now, flip the grip over and put it on the lower and tighten in place. Easy.
I've built a bunch of ARs so feel free to ask whatever.
I've built a bunch of these, lost more pins and springs during the learning curve than I can count. Happy to help, just ask.