An easy way of doing a number of HST (half square triangles}

There are several other ways to quick-piece half square triangle units. It's not a bad idea to try them all to see which methods you like best for different situations, because triangle squares are one of the most common components used in quilts.
Try this grid assembly technique when you need lots of identical triangle squares. By drawing it directly on the reverse side of the lightest triangle square fabric this is really quick and easy

Refer to your pattern or calculate the finished size of the triangle squares. Add 7/8" to that size to determine length and strip width. I tend to add on an inch I can’t be doing with fiddling with 1/8 “, it’s easy to trim it off.

Determine how many squares you need. Each square in the grid produces 2 triangle squares.

To calculate total strip length, multiply the grid size by the number of squares required. Use shorter lengths of multiple strips if necessary to make the best use of your yardage.

Cut strips of the two fabrics chosen for your triangle squares, remember add on the 1” or 7/8”

Use a pencil to mark square segments along the length of the strip. The distance between each vertical line is the same as the width of the strip. Straighten the edge of the strip before you mark to make sure all vertical lines are perpendicular with the top and bottom of the strip.

Draw a diagonal line from top to bottom of first square, next square draw line bottom to top… your making a zig zag line.

Continue marking until all squares are joined by diagonal lines moving in an up and down manner along the strip.

Place your two strips of fabric right sides together match edges. Pin the strips together along their length to keep them from moving apart.

Begin sewing at one end, placing your seams a scant 1/4" from the diagonal line that connects squares. Use your 1/4" presser foot to gauge the distance. Stop sewing and turn at each corner to change directions.

Repeat to sew a seam a scant 1/4" on the opposite side of the original diagonal lines.

This image shows the dark side to show the stitching line as it wont show on the cream side.

Cut units apart on all your pencil marked lines that define your squares. Cut each square into triangles along the pencil mark on the central diagonal.

Place the units on the ironing board, darkest triangle up. Press seam to the darker fabric

Measure units with a square ruler to verify size. The ruler's 45-degree line should flow precisely along the line created by the unit's two triangles.

Trim to required size and to square up if necessary

and if you have only a square of fabric the same principal works mark, grid and sew either side of the pencilled line.

right sides together and stitch either side of the diagonal line easy peasy lemon squeezy