I have a hard time with stippling. You not only have to concentrate on moving the fabric at an even pace but where you're heading with your stitching. Try practicing some easy loops or curves to get your stitch length even. Once you feel good about that move on to the more challenging designs. I like to practice on plain muslin and mark a design to follow. That way I don't have to concentrate on too many things at once. I'm by no means an expert but these are some practice tips that have helped me.