Miter joints are very common, popular joints in woodworking.
They're also devilishly difficult to make. The 45 degree angles need to be cut dead-on to avoid gaps, and the gluing-up process is a horrorshow- the moment you clamp a miter-joint-to-be, the pieces slide around.
Various companies sell a host of jigs and tools for easing the problem. But here's a method that has worked quite well for me, using nothing beyond normal shop tools.
What you'll need:
4 mitered pieces to glue up. You may be able to adapt this process to work on individual joints, but here I'll only talk about gluing up a rectangle all at once.
A table saw, with a sliding rip fence.
a decently straight board, long enough to clamp at both ends of the table.
In truth, you don't really need the tablesaw and fence (you could use a regular table and two straight boards), but it does make life easier, as you'll see.
This Instructable was made at Xerocraft, the Tucson Hackerspace, and is intended to count towards our sponsorship from Instructables.