Have you already cut out your pattern pieces? One way to do this (if all your sizes are listed on the pattern piece) is to start cutting out the top of the pattern at a size 12. As you get towards your waist, start blending (or cutting) towards the 16. As you make your way to your hip then start making your way to the size 18 line. This is perfectly reasonable solution as long as your cut lines look smooth and gradual. Hope this helps! 🙂