HST (Half Square Triangle) is quilting shorthand for making a square out of two triangles. It’s easy, right? Just sew down the long edge of two triangles, open it, and VOILA! you have a square.
Well, almost.
The problem is that the long edge of the triangle is generally going to be on the bias of the fabric. This means that distortion and stretching of the grain will be easy to create during the cutting, sewing, and pressing portions of building an HST.
There is a solution, and here it is.
Let’s say you want to make a couple of 4 1/2″ HST. Cut out two 5″ squares of fabric, one from each of the colors you want your HST to have. Put a little effort into this part, and cut them reasonably square to start with. It will save you some headache later.
Turn one of your squares over so the wrong side of the fabric faces you. Using a ruler, draw a straight line from one corner to the diagonally opposite corner of your square. This line is a GUIDE, not a seam line.
Stack your two 5″ squares right sides together and line them up as closely as possible. Put them under your machine with that guide line you just made lined up with a scant 1/4″ seam. Sew straight down that seam. Turn the square around 180 degrees so that you can sew a scant 1/4″ seam down the opposite side of that line.
Cut along your guide line. Press each square open along seam line, pressing towards the darker fabric.
Put your grid ruler over your new HST. Whew! that’s ugly, isn’t it! Now you know why we made it 1/2″ too big. Time to square up.
Align the 45 degree line of your ruler with the diagonal seam in your square. Scrunch that ruler gently along that line until you have it as close to the corner as you can get it without losing the edges of your square under the ruler. Trim off all excess on both sides.
Turn the square 180 degrees so that the uncut sides stick out. Align the 45 degree line of your ruler with the diagonal seam line again, but this time, make sure that the edges you just cut line up with the 4 1/2″ marks on each side of the ruler. Check that you have three things lined up:

Diagonal seam of square lined up with diagonal line on ruler

4 1/2″ mark on ruler at top aligned with edge of square

4 1/2″ mark on ruler at side aligned with edge of square
Now trim off excess on both edges of the square.
Put your ruler over the square again. You’ve done it! Now you have a perfectly square HST with the diagonal running exactly down the center diagonal of the square. Congratulations, You can HST!