If you need help to sew accurate seams, try using a raised seam guide instead of the flat seam guide on your sewing machine. The edge of your fabric will butt up against the edge of a raised seam guide and therefore not shift easily out of place as it can with a flat seam guide.
You can make your own raised seam guide by wrapping a thick rubber band around the bed of your machine. Or you can layer several pieces of tape together, then adhere this to the bed of your machine.
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