The hardest part of the lower is the front take down pin detent. At least once it will go flying and you will have to search for it. Use two people and a small, thin, flat head screwdriver. Insert the spring first, then the detent. Put the screwdriver into the take down pin hole from the left hand sideand press the detent down with the very tip of the screwdriver and slip the takedown pin into the right side of the hole, pushing the screwdriver out the other side. In theory it will hold the detent and youll be good to go.
It really isnt hard once you figure it out. I havent lost a spring or detent since the second one I put together.