The following steps make approximately 24 2 1/2" HST units. I got 32 because I didn't need to re-align and retrim.
To make 2 1/2" HSTs, cut two fabrics pieces 9" x 21". One of my pieces was a bit longer than 21", and that's okay.
Layer them on top of each other, right sides up, with top, bottom, and left edges aligned.
Using a 6" x 24" ruler, align the 45' mark with the bottom edge of the lower left corner.
Make this initial strip close to 3", measured on the diagonal.
Continue aligning the ruler with the diagonal cut, and the 45' mark along the bottom, to cut 2 1/2" strips all along the piece.
Now for the fun part!
Take the top piece off the first strip, set it above all the cut pieces. Take the bottom piece of the next strip set and lay it next to the first piece. Keep alternating the strips to get these two units
When done, gently press the seams to the side of the darker fabric. And I mean PRESS, don't iron! If the iron moves, it will distort the bias seam.
Using a 10 1/2" square ruler, line its diagonal mark up with one of the center seam lines. Place the 6" x 24" long ruler along the left side of the square ruler, just over the fabric.
Move the square ruler up and down alongside the long ruler to double check the diagonal seams are still within 45'. Remove the square ruler, and cut along the long ruler. It's okay if one end has more trimmed off.
Align the 2 1/2" mark on the long ruler with the cut edge, and cut again. But before cutting another 2 1/2" strip, place the square ruler back on the unit, aligning the diagonal, just as before. If the unit needs trimming on the left again to make the seams at 45', then trim as necessary.
This is the area I did not need to retrim, thus allowing a few more HSTs.
These are the strips after cutting.
Two sides of the HSTs are now done. Now for the rest.
Align a small ruler with the right edge at the bottom seam point. Align the top of the ruler with the rest of the strip, and the 2 1/2" mark with the top seam point.
Trim off the excess. Move to the next group of seam points, and repeat, continuing to cut the strip. Turn the resulting pieces 180' to trim the other side, making them 2 1/2" HSTs.
The last sequence is in the picture below, bottom to top.
Perfect, easy 2 1/2" HSTs!!!! I love this method! The next time I have to make a bunch, I'm doing it this way. And I don't see why scraps couldn't be used, as long as there's enough for a bias strip in the initial unit.
And to make different sizes, cut the bias strips 1/2" wider than the finished short side of the triangle in the HST. But if the HST finishes larger than 3", cut the bias strips 1/4" larger than the finished short side.
Have a Happy Day Quilting!