Look at the stitch numbers given for the different sizes, usually there's about 6, or 8 sts difference in them. Find out how many sts per inch you're going to knit at, multiply that times 30 (or 32 for a loose fit, 28 for a snug fit) and that's how many sts you need to fit at the largest point. Then see if that is one or two sets of 6/8 sts less than the smallest size. Use that ratio everywhere in the pattern where a st number is given.
I hope I've explained that clearly enough, ask if you don't understand something.