There are many, many ways to create Half-Square Triangles(HSTs): the grid method, Triangles on a Roll®, Thangles®, a number of rulers, templates and more that I haven't thought of...the way I describe below is my prefered method of doing HSTs.

To calculate the cutting size of the initial square, add an additional 1/2" to your cutting size for a square unit. For example...for a 12" 9-patch square block, you would cut your squares 4-1/2"...to cut a square to create an HST, you would cut 4-1/2" plus 1/2"...a 5" square. Once you have cut all your squares:

1. On the back of the lightest fabric, mark a diagonal line.
2. Match the opposite color with the marked square, right sides together.
3. Stitch 1/4" on one side of the diagonal mark...turn square and stitch 1/4" on the other side of the diagonal mark.
4. Cut apart on diagonal mark.
5. Press seams open toward darker fabric.
6. Square up completed unit to size needed (ie...if you started with a 5" square, trim to 4-1/2" square)

The quilting books say that you only need to add 3/8", not 1/2", to the measurements, but I find that stitching and pressing fabric on the diagonal is not an exact science for me. Regardless of how many I have to do, and because no matter which way I do them, I can't make consistently sized units to save my life! VBG!!! This way will use just a little more fabric, but will allow you to control the final measurement of the unit.