Easy as pie.
The only part of assembling a lower that can be screwed up is putting together the trigger guard- the receiver can be ruined if you go about it hamfisted.

Here's a very good guide to putting together a lower along with rebarreling an AR-15, I like their method of using vise grips for pushing some of the pins home- sometimes the pins fit very tight and are difficult to drive in with a hammer and pin punch:

http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782

Putting an upper together is very easy as well, but you need a few special tools (upper receiver vise block and barrel nut wrench). The most difficult things to do that require a machine shop (indexing, drilling and pinning a front sight tower, and headspacing) are almost always done already when you purchase a barrel. You can have a chamber custom cut and headspaced to a bolt, but it isn't necessary as AR-15 bolts and factory headspaced barrels are interchangeale.