First off make sure the gun is unloaded.
Close bolt on empty chamber.
Push out rear takedown pin to tilt upper out of the way.
Look in the back of the lower where the buffer tube threads into the lower.
In the bottom of the threads there is a hole these two parts, spring and buffer detent, go into.
If they will not go you need to back the buffer tube up in the threads by turning it counter-clockwise from the back until the parts have room to fall into the hole.
Spring first then detent, nipple pointing up.
While turning buffer tube clockwise to advance it in the threads use your thumb or something to hold a little pressure down on the detent so you can advance the tube up to the nipple on top of the detent, but not quite touching the nipple on top.
The buffer tube actually hold the spring and detent in the hole.
Hope this helps.
Clayman