For 2 1/2" finished half square triangles start with 3" squares. Take the Omnigrip ruler and just put a small mark at the top left hand corner where your 1/4" seam would be start. (Sorry for the glare but without the flash the picture was way too dark.) The central line from which you are measuring the placement of the mark is the diagonal line from corner to corner of the fabric. Cathy actually has made so many of these that she no longer even marks her triangles, she just knows where to place them. I guess when you've made thousands of them you have it figured out.
Take your two pieces of fabric (right sides together) and line up the needle with the mark that you made and line up the bottom point of the fabric with the edge of the painters tape and sew. Keep an eye on the bottom point to ensure it keeps following the edge of the painters tape. You can chain piece all your triangles this way. Once you have done one side, just repeat on the other side (mark starting point and line up with needle, line up bottom point with edge of painters tape and sew ensuring that bottom point follows edge of painters tape). Cut and press triangles as usual.
To trim triangles there is this handy little 2 1/2" Omnigrip ruler. Again sorry for the glare. It took a bit to getting the hang of using it, but once you got it, it does make the trimming process easier than with the bigger rulers. The ruler has a diagonal line marker and you place it along the diagonal of your half square triangle and line things up before cutting.