1. Prepare the Zipper Opening

• With right sides together, stitch the seam below the zipper opening at your pattern’s seam allowance (typically 5/8 in), backstitching at the zipper stop point.
• Machine-baste the zipper opening section using the longest stitch length on your machine (this will be removed later).
• Press the entire seam open, including the basted zipper opening section, creating crisp folded edges on both sides.

2. Position the Zipper on the Underlap Side

• Place the garment wrong side up with the pressed-open seam visible.
• Identify the underlap side (typically the left side for center back or side seams; check your pattern).
• Position the closed zipper face down on the underlap seam allowance with zipper teeth centered directly over the basted seamline.
• The zipper stop should align with the marked zipper opening end point, and the zipper tape should extend into the seam allowance.
• Pin or clip the zipper tape to the underlap seam allowance only, keeping the zipper teeth centered on the seamline.

3. Stitch the Underlap Side

• From the wrong side, attach your regular zipper foot and adjust it to stitch close to the zipper teeth (right side of the foot for this pass).
• Starting at the bottom of the zipper, stitch through the zipper tape and underlap seam allowance only, stitching approximately 1/8 in from the zipper teeth.
• Stitch to the top of the zipper tape and backstitch.
• This stitching will not be visible from the right side and anchors the zipper to the underlap.

4. Fold and Pin the Overlap Side

• Flip the garment to the right side.
• Fold the overlap side (typically the right side) so the pressed fold extends approximately 3/8–1/2 in beyond the seam center, completely covering the zipper teeth.
• The basted seam should remain closed, and the zipper should be hidden under the overlap fabric.
• Pin the overlap fabric in place from the right side, catching the zipper tape underneath, with pins perpendicular to the fold.
• Optional: Use basting glue stick or Wonder Tape on the zipper tape to hold the overlap precisely in place.

5. Mark the Topstitching Line

• From the right side, use a fabric marking tool and seam gauge to mark a straight topstitching line parallel to the folded edge, typically 3/8–1/2 in from the fold (or as specified by your pattern).
• Mark a horizontal line at the bottom of the zipper to create the characteristic lapped zipper box shape at the bottom end.
• The horizontal line should extend from the topstitching line to the seamline, creating a small rectangle or triangle at the zipper bottom.

6. Topstitch the Overlap from the Right Side

• Attach your regular zipper foot and adjust it to allow stitching along the marked line (left side of the foot for this pass).
• Starting at the seam below the zipper, stitch across the horizontal bottom line, pivot at the corner, then stitch up the vertical topstitching line to the top of the zipper opening.
• Backstitch at the beginning and end.
• Keep the zipper pull partway down to avoid stitching over it; stop with the needle down, raise the presser foot, slide the zipper pull past the needle, lower the foot, and continue stitching.

7. Remove Basting and Finish

• From the wrong side, carefully use a seam ripper to remove the machine basting stitches from the zipper opening.
• Open the zipper and check for smooth operation; the overlap fabric should lie flat and completely conceal the zipper teeth when closed.
• Press lightly from the wrong side, using a pressing cloth to avoid melting the zipper teeth.
• Optional: Hand-tack the bottom of the zipper tape to the seam allowance below the zipper opening to prevent flipping.

8. Troubleshooting Common Issues

• Zipper shows when closed: Increase the overlap width or reposition the topstitching line farther from the fold.
• Rippling fabric along zipper: Add interfacing to seam allowances or reduce fabric handling; press more carefully.
• Topstitching is wavy: Mark your stitching line clearly and use a seam gauge or edge-stitching foot for consistent spacing.
• Zipper won’t open smoothly: Check that you haven’t caught extra fabric layers in your stitching; restitch if needed.
• Fabric caught in zipper: Ensure seam allowances are secured and trimmed; backstitch securely at the bottom stop point.