1 If you plan to puddle your curtains, you only need to ensure you have plenty of material left over for the puddle. 2  Curtains that hang below the sill should extend 4 inches (10 cm) below the sill, long enough to cover the edge of the casing below the window.3  Sill-length curtains should skim the sill.se a measuring tape. Determine where the top of the curtain will be from where you want the curtain to end: at the sill, below the sill or on the floor. Bear in mind that some styles of curtains have ruffles or pleats that extend above the top of the rod; subtract this section from the measurement.