Measure height and width of the inside frame of your window. Add 1 1⁄2 inches to width (to allow for side hems) and 3 inches to height (to allow for top and bottom hems and curtain-rod channel). Measure, mark, and cut out fabric.
Fold sides and bottom edge of fabric under by 3/8 inch; press with iron. Fold under by 3/8 inch again; pin and edge-stitch (stitch around the three sides, close to edge of fold). Fold top edge of fabric under by 3/8 inch; press. Fold under by another 1 1⁄2 inches (to create channel) and edge-stitch.
Cut an 18-inch length of twill tape. Fold in half, to a 9-inch length. Pin at fold at bottom inner corner of curtain. Stitch in place by hand.