Here's a clickable link:
http://www.clotilde.com/detail.html?prod_id=9336
And WOW, that ruler has a heckava price tag.
Here is the youtube video from Sharon Schambers on mitering corners. She's a master quilter, and it's free. ;-)
Worth way more than $30.
Part 1
http://www.youtube.com/watch?v=zAAa66kJRr0
Part 2
http://www.youtube.com/watch?v=UJhraFK3zSY
Part 3
http://www.youtube.com/watch?v=jZWNCcA2RUU
Part 4
http://www.youtube.com/watch?v=fsrr1Ep2sjo
You don't have to do EVERY single thing she does, but it will certainly help you understand the entire process.