Sew-in interfacing should be preshrunk by the same method of care that will be used for the finished garment. Dry-clean only interfacings can by preshrunk by steaming with a steam iron and letting it air dry before use.
Sewers usually remember to preshrink fabric, but often forget about the interfacing. It is necessary to preshrink interfacing to avoid the distortion and puckering that occurs when the finished garment is cleaned. Preshrinking fusible interfacing will eliminate the bubbling that can occur during the fusing process.
To preshrink fusible interfacing, place loosely folded interfacing in a tub of hot water and let it soak until the water is cool. Carefully squeeze out excess water by gently pressing it between two towels; do not wring. Drape over a shower rod or towel rack with the fusible resin side up and let air dry.