First, my quilts/stitching are far from perfect. I’m ok with that. However there are tricks to help.
1) accurate cutting. Use good rulers, investment. Sharp blades, investment. And fabric with enough body so it’s not flimsy.
2) make sure the seams are 1/4 inch. ‘Power Cutting”, a book, has great teaching on achieving this for straight edge shapes. A barrier on the machine bed can help. This is an old credit card or stack of masking tape adheared at the 1/4” mark. On my 9mm wide machine, it sews more accurate seams with a standard foot and move the needle 2-3 clicks to the right.
3) Use a 50 or 60 wt thread
4) pressing (not ironing) with NO steam.
5) a note on machines, a straight stitch only machine does a more accurate straight stitch than a zig zag machine. It has to do with the motion of the hook, ascillating or rotating.
Relax. I think this is suppose to be fun?
#2)….cut 3 1.5inch x 4inch strips. Easily differentiated values will help this process. Sew them together on the long sides. Press with seams open. Measure. It should be 3.5 inches. If not, try again, adjusting width of seam or weight of thread. Keep trying until correct. Remember this isn’t rocket science and doesn’t need rocket science accuracy, it’s not going to hit the moon.
Again. Relax. Have fun.