I have the Quilt in a Day ruler where you trim before you open up the square and like that when I remember to use it. Otherwise I just use a square ruler and put the 45 degree line along the line where the two sides are stitched, trim two sides, turn trim the other two. The quilt in a day ruler you only have to trim two sides and takes less brain power in positioning it. I've never had trouble with it slipping but you could use the sand paper dots to help.
If I'm going to make a lot of hst then I use a software program called triangulations. It comes on a CD through Bear Paw. It prints out templates that you sew through the paper. Works great and very accurate. Here's a link to the site:
https://www.bearpawproductions.com/s...php?cat=1&ID=2 It's also available through Amazon.