Here's a video at Long Arm University.
http://www.longarmuniversity.com/Boo...g_tutorial.htm
I am just using a regular ruler (I ordered that one) and using the ruler to make straight lines from corner to corner, from corner to seam, corner to corner, etc. Do it one way then come back and do it the other way.
Just have to remember to place the ruler 1/4" away from the corner that you want to hit because of the width of the foot.