If you are just starting to stipple and are tense about crossing over the lines as you go, then print a stipple design on a large piece of paper, take the thread out of your machine, lower feed dogs and follow the stipple lines on the paper with your needle. This will get your hands, arms and movement into the stipple motion. Practice this, then practice on fabric. Place a stipple design sheet under some light fabric, draw the lines on your fabric and then go to the machine and follow the lines. Once your hands, foot and eye can follow the lines, they will remember that motion when you are free motioning later.