I recommend always cutting your squares 1″ larger than the finished size. IE. if your half-square triangle unit finishes at 1 1/2″, you would cut your squares 2 1/2″. This allows for just a little room to straighten or square up your half-square triangle before sewing it into the block.
Traditional method, my grandmother might have taught me:
Starting with two squares of equal sizes, cut both on the diagonal. Take one from each color, place them right sides together, pin, pin, pin and sew on the long bias edge 1/4″ from the raw edge. Press to the darker side. This method does come in handy when you are working with lots of scraps and want that scrappy look.
A more modern method:
Starting with two squares of equal size, draw a diagonal line from corner to corner.
Sew 1/4″ on both sides of the drawn line.
Cut on the solid line and press to the dark side. Square up to 1 1/2″.
To square up your half square triangle, use your favorite square up ruler. I like the Precision Trimmer, however a 6″ square ruler with a 45 degree also works nicely.
Lay the 45 degree across the diagonal line, leaving a small amount of fabric around the square. Right hand – rotary cut up the right side and across the top. Left hand will cut up the left side and across the top.
Rotate the unit so that the right side and top now sit in the 90 degree angle on the bottom left edge. Once more, rotary cut up the right side and across the top. Your half-square triangle will now be squared up and ready to sew into your block. Left hand will cut up the left side and across the top, rotating to the right bottom corner.
Using Triangle Paper:
Quilters love the Finished Triangle Paper, which comes in many sizes, is simple to sew, accurate to cut, and effortless paper removal.
Cut your squares to the appropriate size based on the directions that comes with the Triangle Paper. Place the two pieces of fabric right sides together and lay the Triangle Paper on one of the wrong sides.
Sew on the dotted lines, Cut on the Solid lines. You will need to adjust your stitch length to 1.0 or 1.5 when using Triangle Paper. This makes it so much easier to tear away the paper after cutting. Press to the darker side.
The not so proper way to do a half square triangle.
Oh, no, please don’t put those bias edges on the outside of your blocks. We have seen a “quick” method of placing two squares together, sewing around the outside edges and then cutting both directions on the diagonal. This places those bias edges on the outside of your block, making it a challenge for sewing into a quilt. This method is not for a beginner.