AR manufacturer CMMG has an animated video showing how to assemble an AR lower. It is actually a pretty good video. That said, nothing is quite as easy as it looks. At least not for me!
I first saw the video on the ENDO Gun Blog. If you have put together your own AR-15, then you need the T-shirt to prove it! Actually, I'm wearing mine right now and its a very soft, comfortable shirt.* You can buy it here.
*For the FTC, I bought the shirt a few weeks ago and I paid full price. There, are you happy now?