Lay out the fabric you are using to make the drapes on a flat surface, wrong side up. It should already be cut to the dimensions needed.
Fold the sides of the fabric over half an inch. Also fold the bottom edge of the fabric up half an inch. Iron the folds to create a crease.
Fold the sides over another half inch. Fold the bottom up another half inch. Press with the iron.
Unfold the sides and bottom. Draw a diagonal line from the outer crease on one side to the outer crease on the bottom edge. Repeat on the other side.
Trim away the fabric along the diagonal line so that you cut away the point of the corner.
Fold the sides and bottom edges over again, along the creases. Pin them in place, then sew along the inside edge. Finish the rest of the drapes per your design or pattern.