I promised I'd post directions for making 12 half square triangles (HSTs) at a time, so here it is. This method is based on one I saw somewhere else for making eight at a time. I liked that method, but not having to stop at each corner, then come back to the fabric, so I adapted it to 12, so I could do continuous lines of stitching.
To start, decide what size you want your HSTs to be. (Mine are 6.5 inches.) Add one inch. (that's 7.5). Double that for the short dimension of the rectangle (mine is 15) and triple it for the long dimension (mine is 22.5.
Cut two rectangles in this size, in the two fabrics for your HST. Lie the lighter fabric face down on your work table. With a fabric pencil, mark one short end at the halfway point (7.5 inches for me), and mark both the longer ends at that distance from the end you just marked. Draw lines from the first mid-point to the points on each side, then from those points to the opposite corner, as shown in the picture.
Starting in one corner, stitch 1/4 inch from the marking line on both sides of the lines. For the outside stitching, you can stitch off onto a scrap of fabric, pivot, then stitch right back on the next line.
After stitching both sides, cut vertically and horizontally first, then cut along the marked lines. Press the HSTs open, then use a square to trim to exact size.
Super quick HSTs, ready to go.