First of all, don't sew any part of the seam where the zipper will go - leave the whole seam unsewn.
Also, choose a zipper that is about 5cm longer than the zipper opening. This allows the zipper end to be pulled out of the way when completing the garment seam.
- Interface both sides of the seam with long strips of iron-on interfacing.
- Lightly press under 1.5cm seam allowances in the zipper area. This fold line provides a guide for the zipper placement.
- Lay the garment fabric pieces wrong side up with the pressed seam allowances lined up.
- Position the zipper with the right side down over the lined up fabric seam allowances. Mark the top and bottom (not the metal stopper but 5 cm up) of the zipper opening on both the seam allowances and on the zipper tape.
- Open the zipper. Unfold the seam allowances and position one row of zipper teeth along one folded out seam allowance, matching marks on the zipper tape and seam allowance. The teeth should lie exactly over the foldline, not inside. Pin in place using lots of pins. Don't pull the tops of the zipper into place, let them splay outwards.
- Sew the invisible zipper into place from the top down, through the seam allowance only, using a slightly looser top tension in your seewing machine and guiding the teeth feed through the foot channel. Sew as far as the lower mark on the bottom of the opening (not to the bottom of the zipper).
- Sew the other side of the zipper in the same way.
- Close the zipper.
- Place the garment pieces with right sides together, and the zipper sandwiched in between, and pin the seam below the zipper opening. Sew the seam from the bottom up using a normal zipper foot. About 2cm below the zipper opening, shorten the stitch length and overlap the stitching line of the invisible zipper for about 1cm.
- Hand bind the teeth at the bottom mark of the zipper, effectively creating your own stopper, and chop off the rest of the zipper. If you like, machine stitch the lower 2cm of the zipper to the seam allowances.
- Lightly press the closed zipper from either side, on the wrong side. On the right side, steam and finger press the zipper seam in place.