So this was fairly straight forward and easy, and it turned out that I actually had a pattern (and a Gramma) to help, but it was pretty easy... and SO much cheaper than buying one... woo hoo!
Step one: Measure 45" by 69" and cut it.
Step two: Fold corner diagonally and measure 9" sides (only on 2 sides) and then cut.
Step three: Take the corners and put right sides together and stitch.
Step four: fold and press 1/4" and then 1/2" seam.
Step five: Hem it all the way around leaving about 2" somewhere (not on a corner) so you can stick the elastic through.
Step six: put a safety pin on one side of the elastic and feed it all the way through (I used 3 yards, but didn't end up needing that much). Sew the ends of the elastic together when it's the right length.
and VOILA! Your done. Easy Peasy.
If my bulbous pregnant bellied self can do it... I promise, so can you!