the ar-15 is one of if not the most simple center-fire high-power semi auto to work with there is.
there is not a whole lot of smithing to be done really.
the only parts of it that are much work are.
setting up a barrel for the GI front sight post. this requires some steady measurements and some drilling. a mistake and you get off the barrel port and or a canted front sight.
installing the barrel in the upper, this requires the barrel nut to be tightened up and in time with the gas tube. generally pretty easy. but many first timers bend the gas tube.
and then there are those little flea sized retaining pins that capture the big pins in the lower.
and there is the springs being installed backwards or getting swapped around.
all but the last two are if your building an upper.
if you buy an assembled upper all you will have to do is install the stock hardware, install the LPK and then attach the upper on the lower.
DPMS even has the parts mapped out on their skin-packed LPKs so it is sort of like having an instruction manual.
there is an army/AF manual on the M-16 that has about all you'll need to know about building any ar-15 in detail. has drawings of where the parts go and what to look for.
i think it is in the downloads section here. a good read for anybody wanting to build the ar-15.