Take a look at your feed dogs to see if they are pulling the fabric through straight. Otherwise I think practice and attention to detail might be the answer. Use a small wooden chopstick or a little stick to help guide your fabrics through. I like the wooden ones because if you should happen to hit your chopstick it won't break the needle or throw your machine out of time. My Baby Lock machine has a 1/4 inch setting which I use, however it is not EXACTLY 1/4 inch. Go figure! It works for me though so I use it all the time. Usually consistency is ore important than being exact.