Wash and press your fabric
Lay the fabric out on a flat surface. Cut the fabric 11 inches wider than your window and 4 inches longer than you want the curtain to be.
Turn the sides of the fabric under 1/2 an inch and topstitch. Press the hems.
Turn the top of the fabric over 2 inches. Fold the edge under 1/2 an inch. Topstitch the fold. Topstitch another line just below the top fold, leaving enough room for the curtain rod to fit inside the pocket this creates. Press the hem.
Turn the bottom of the fabric up 2 inches. Turn the edge of the fabric under 1/2 an inch and topstitch the hem. Press the hem.
Cut two lengths of ribbon twice as long as the original length of the fabric. Fold the ribbons in half. Mark the center point. Fray check the ends of the ribbons.
Fold the curtain in thirds, lengthwise.
Pin the center point of the ribbons at the top of each of the folds.
Open the fabric up. Pin the ribbons so that they hang evenly on either side of the curtain. Topstitch the ribbons at each of the stitch lines for the curtain rod pocket.
Thread the curtain onto the curtain rod. Space the curtain across the rod so there is a gentle gather.
Mount the curtain rod on the window according to the curtain rod instructions.
Tie the ribbons in a bow at the base of the curtain, gathering the fabric to the desired height to give the balloon effect.