I should make some tutorials.
Some folks have asked before how to do structural things, particularly in Flash...
This is pretty much it in a nutshell. Two basic techniques here:
Cross a line with a helper line to add a corner point to it. Cross two lines and delete one leg to make a 3-way join.
And it's easier to make the intersections you need with the pencil tool then straighten, than to try to use the line tool for EVERYTHING.