Funny I have more problems with pinstripes than widder stripes, dont know if it is the right way to do it but we always run a line of masking tape along where the stripe is to go, you can then check along the masking tape line from one end to see if it is straight, if not make any adjustments. When you are happy the masking tape line is OK you then have a guide to run the vinyl along. Seems to work for us but dont know if there is an easier or better way to do it. We tried pouncing a chalk line but the line kept coming out quite wide as the chalk spread out and this made it quite hard to follow.