First you start with 2 squares, one colour for one half triangle. Mine are Christmassy red and white.
Unfortunately we need math. Don't worry, it's simple:
Meaning: A is the unfinished hst and B your starting square.
My hst is 3 1/2". Add 1/2" makes 4". x2 makes 8. So my starting square B was 8".
For safety I added another 1/4". If you are just as accurate as Harriet Hargrave you need not do that.
Now what to do with these? Make a Christmas pillowcover that's easy to put on and off your cushion!
Sew your block of choice together and put a piece of thick fusible fleece on the back and quilt it if you want.
Then the back. Same hight as the pieced block but for the width, cut a piece that's approx. 1 1/2 the size of the pieced block.
Sew a lockstitch all around to prevent any fraying.