Accuracy takes practice. Both in cutting and sewing!
It helps to:
-use a non slip ruler (watch some videos on how to cut accurately with rulers...there are tips and tricks)
-prewash and press your fabric...spray starch also helps
-use very fine pins when pinning to minimize distortion issues (I like the Clover Fine Patchwork pins)
-use a needle plate with a single small hole for the needle, not the zig-zag plate
-see if your feed dogs feed evenly...many don't hold the fabric straight enough as the seam ends (hold the end in place as you sew if that is the issue)
-put a 1/4"guide down on your machine bed, either layers of masking tape or a sticky on one side vinyl or plastic guide
-use "leaders and enders"
https://www.allpeoplequilt.com/how-t...erfect-14-seam