If you want another source, Brownells has a nice how-to:
http://www.brownells.com/.aspx/lid=11011/learn/
If that link doesn't work (and it's iffy depending on what you're browsing on), go to the Desktop Site if you're on mobile, click Learn, then find Build an AR. There's a section for assembling the lower.
7 short video clips, plus a PDF (for each clip) you can print out. I still grab the PDFs I keep store away with the tools, even after building a dozen or so.
ETA: forgot, the buffer tube and stock are in the next section, two sections here:
http://www.brownells.com/.aspx/lid=11012/learn/