http://quiltinaday.com
Go to the tutorials on rulers and watch the video. It is in the television and video section.
You can do it without the ruler but it is so much faster with it. The ruler is at Joanns so get it with a coupon.
The ruler comes with instructions for all sizes up to 6 1/2 inches. It also has instructions for quarter triangles.
Basically you set the line for the size HST you want on the stitching line and make one cut.
Best part they are perfect every time.