You will be given instructions to cut pairs of squares. I usually give generous measurements.
The rule for working out the size of the squares to cut to make Quarter-Square Triangles:
Add 1 ¼” to the required finished size.
Make a pair of Half-Square Triangles as per the tutorial. Press them, however, don’t bother trimming them to size.
On the back of one square, draw a pencil line at 90° to the existing seam, on the diagonal:
Butt the central seams:
Sew a scant ¼” seam either side of the diagonal pencil line.
Then, cut the seam on the pencil line:
You have now created two quarter-square triangle blocks. Press the seam without opening the fabrics, to ‘set the seam’; then, open out your block, and press towards the darkest fabric.
Now, trim the square to the exact size. Firstly, line up the diagonal (the 45° angle) on your ruler with the seam. Secondly, position the ruler so that the mark that represents half the unfinished finished size is exactly in the centre of the block. This is very important!