Cut two strips of plastic material for the zip tabs, each the same width as the zip and 3” long. (This assumes the zip length is 2” less than the width of the rectangle you cut. Each zip tab will fill half the 2″ gap, but it will be folded in two and have a ½” seam/turning allowance at its ends. Cut different length tabs if necessary, or just fold under a bit more or less at the ends. My zip measured 7½”.)

Turn under ½” at each end of each zip tab and crease the fold sharply by running something like the edge of a ruler along the fold — use a bone folder if you have one. Fold a tab around each end of the zip to encase them — open the zip a short way to make this easier at the top end. Hold the tabs in place with clips, you can’t use pins because that will make permanent holes.

Now it’s time to sew. Practise first on a scrap piece of the plastic material to find out the best needle and tension to use. I had success with a fine (no. 80/12) sharp-pointed needle and my usual upper thread and bobbin tensions, but I used a bigger needle for the zip tabs to be sure it would cope with stitching across the zip teeth. Use a relatively long stitch (at least 3mm or 8 per inch) or the plastic may rip along the stitching line.

Stitch across the first zip tab, close to the fold. As long as the zip has nylon spiral teeth you should be able to sew across them, but take it slowly. With the second tab clipped in place, lay the zip complete with its tabs on the rectangle of plastic that will form the bag’s outer to check that it is now the same width as the outer (and lining). Adjust the turnings on the second tab if necessary before stitching across it as before.

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