Prewash before you cut. Most cotton knits, including French terry, shrink a little the first time they get wet and dry, so wash the yardage the way you plan to wash the finished garment before you cut your pattern. For French terry: machine wash cold or warm on a gentle cycle, then tumble dry low or line dry. High heat can tighten the knit and fade color, and turning garments inside out reduces surface pilling. Wash matching rib with the body fabric so shrinkage stays aligned. Then cut — and expect the same care after sewing if the piece will be washed often.
Read the full guide: Fabric Care for Fabric by the Yard