Measure your lace curtain for width and drop. Measure down to the bottom of the scallops if you want your lining to also have scallops. Measure down to the top of the scallops if your lining will end above the lace scallops.
Cut your lining fabric to the dimensions of the lace curtains, adding 1 inch to the drop measurement and 2 inches to the width measurement.
Pin the side edges of the lining material under by 1/2 inch, then 1/2 inch again toward the wrong side of the fabric. Stitch the side hems down to neaten the edges.
Turn the top edge of the lining fabric under toward the wrong side by 1 inch and stitch to neaten the top edge.
Lay both layers of fabric on a flat surface with the lace curtain on top of the lining. Overlap the lining with the lace at the top edge, so the edge of the lining is level with the bottom of the hem along the top of the lace. Line up the sides of both fabrics.
Trace the outline of the scalloped lace edge onto the lining fabric. Use tailor's chalk or a washable marker.
Lay the lace curtain flat on a large surface, right side up. Lay the lining fabric on top of the lace, right side down, lining up the scallops. Pin the two fabrics together along the side edges. The top of the lining should be just below the hem at the top of the lace.
Sew the side seams of the two fabrics together on the sewing machine, leaving a 1/2-inch seam allowance.
Turn the lined curtain the right way out, and then stitch the top of the lining to the lace along the existing lower line of stitches on the hem. Sew only along the straight sides, not along the scalloped edge.