Thank you Ceil and Cas; I'm going to try both ways!  I thought I was doing something wrong when I kept chipping the shape cut ruler.  I'll try the smaller rotary cutter on a different slot and see what happens.  I'll also try a practice one in the 17" size to see if it still works after it chipped.  I also didn't start with a square; I tried folding it with the rest of the fabric hanging down.  I don't know why I didn't think of cutting the square first!

One more question if I may:  is there a seam guide specifically for Janome?  I bought one for my Babylock, but I can't seem to get the screw to fit in the hole.  Or maybe I'm just doing something wrong - I spent an hour trying to change back to the regular presser foot holder before I realized  I was trying to use the wrong hole!  Sue

P.S.  If all the previous messages appear in this post, could someone tell me how to delete them?  I know it gets very confusing to re-read them all, but I don't see them in my reply, so I can't delete them.