When I make HST's (and QST's), in addition to washing and starching, I over cut my starting blocks so that I can square them up. The general rule of thumb I use is to make the square 1/4 to 1/2 inch bigger than the pattern calls for. I can then cut the sewn blocks down to size.
Starching is important (even if you don't prewash) because if the fabric is stiff it doesn't stretch as easily.
The classic way to check your seams is to prepare your scrap fabric the same way you do for your quilt (if possible, us the same fabrics you are using in the quilt), cut 3 2.5" squares and sew them together then measure your finished strip - it should be 6.5 inches. If it isn't, adjust your seam allowance accordingly and try again.
If you are not using super thin thread, the thread may be the reason the blocks are smaller and that is when you need to use a scant 1/4" seam. Also, one of the fabrics may be slightly thicker than the other and that will throw things off. As scant 1/4" seam means just a couple of threads smaller than 1/4". If you can adjust your needle a bit, that is the easiest way. I use a note card with lines in it (the lines are 1/4" apart) and I mark a
Last edited by QuiltnLady1; 10-01-2022 at 01:16 PM.