Rulers designed for Half Square Triangles are a great help in creating these units. Shown here is Onmigrid #96 ruler in the 8 inch size. It also comes in a 6" size. Another ruler that works the same way is the Easy Angle Ruler. Be sure to study the markings on whichever ruler you use so you understand what they mean.
Cut strips 1/2" wider than the finished size of your units. In this case I am making 4" half square triangle units, so the strips were cut 4 1/2" wide.
Place strips right sides together and straighten the end. Then position the ruler for the first cut. Note that the edge of the strip is at the 4" line on the triangle ruler -- this tells us the FINISHED size of the unit. Also note that the top dog ear is missing.
We have made the first cut and the ruler has been turned around for the second cut. The 4" line is now on the top edge of the strip and the "missing" dog ear is at the bottom.
Two cuts have been made. Ruler is lined up for the third cut. Continue across the strips.
The second dog ear can be removed at this stage. (Note that the cut triangles line up with the 4 1/2" lines on this ruler as it is showing us CUT size, not finished size.) The dog ear sticks out to be easily nipped off with the rotary cutter.
Stitch the seam. You need an accurate 1/4" seam allowance for this step -- there is no "fudge factor" to be trimmed off later!
Here is our finished square. Note that it lines up at 4 1/2" on the regular ruler, and the diagonal goes exactly from corner to corner. The dog ear can be trimmed off at this point if you didn't do it before sewing.