Using a Sewline pencil, rule a diagonal line on the light square. I like to use a sandpaper board to keep my fabric stable.
Place the two fabrics right sides together, and using your 1/4" foot, machine sew down each side of the ruled line.
Press to the dark side. Trim dog ears to reduce bulk if desired.
Some of you will be beyond this information, but I also have to consider new quilters who may not have had experience with this technique. Many people say the key to these is to add 7/8” to the measurement, e.g. if you want half square triangles to match a 5” square, make your squares to do the triangles 5&7/8”. Mostly I think this is for the folk that like to then cut their triangles down to size. I have just made many 3” ones, cutting the half square squares to 3&3/8”- worked perfectly and no need to cut down.
My tip for making many of the same type of block is to trial one, then make sure that is accurate before you cut the quantity needed for your project.