It is a moderately easy job to sew in a separating zipper. Measure the garment, determine the length of the zipper and pin it in place. Use your sewing machine with the zipper foot to complete the job.
Measure the front opening of the garment. Purchase a separating zipper that is at least 2 inches longer than the opening.
Close the zipper and pin one side to the facing of the garment, right sides together. Fold the facing in place and adjust so the teeth of the zipper are next to the seam line. Use the zipper foot and stitch down the length of the zipper and facing. Turn the facing and iron it flat.
Place the right side of the garment under the zipper foot; sew a seam line from top to bottom of the garment, close to the teeth of the zipper. Repeat on the other side of the garment.
Turn the excess zipper at the top so that it is wrong sides together on the inside of the garment. Hand stitch the loose ends to the back of the zipper so there is no raw edge at the top of the garment.
Open the zipper. Run a perpendicular line of stitching through the top teeth on each side of the top of the garment. This allows the zipper to stop exactly at that point.