If your seam allowances aren’t even all the way through your project, you may end up with wonky shapes!
Keep things nice, clean and even by using the .5mm (0.25 inch) seam allowance on your machine. If you don’t have one, measure .5mm/0.25 inch with a ruler and mark it with a bit of masking tape.
If you’re sewing by hand and not confident to eyeball it, draw the sewing line using a ruler and chalk or a water soluble fabric pen.
The extra time spent preparing your sewing lines is well worth it in the long run!