Accurate cutting is step #1 & cutting on the straight of grain for square blocks. Then you have to be able to sew a straight line & make all your seams the same 1/4", 3/8", 1/2", whatever--as long as it's the same on every block. Pressing seams so they alternate helps lock your patches, blocks, & rows together and that locking helps keep everything square.
Starch &/or good quality fabric will help keep things square also but if you cut off grain to begin with, the edges will become unstable and distort. If you use good quality fabric & cut it straight, you still have to be able to sew a straight seam.
Don't feel bad. I threw more blocks in the trash than I kept when I first started. I even tried to fussy cut diamonds for a Tumbling Blocks quilt and it looked like I had sewn B cup bra's together. I even knew about bias from sewing for clothing to drape & give like I want but for some reason I didn't think bias applied to quilt blocks.