This method for a concealed zipper can be used for cushion covers, clothing and more.
How to sew a hidden zip: in pictures
Sew a zip in place using the concealed method. You will need a normal and invisible zipper foot
Note the zip is inserted into the seam first and then the remainder of the seam is completed.
1. The concealed method of inserting a zip works best with thicker fabrics because the seam allowances hold it in place. If the fabric is thin, apply interfacing to the seam allowances first to reinforce them. To do this, cut a strip of iron-on interfacing to fit the length of the zip slightly wider than the seam allowance and apply to the wrong side of the fabric. Make sure your iron is on the correct setting.
2. Open the zip and tack the zip tape in place to the seam allowance. You can tack it by hand or with a machine stitch. We used a machine stitch in this step. When machine tacking the zip tape in place, take care that you don't catch the zip teeth into the seam. And use a matching coloured thread since this row of stitching will remain in the seam.
3. Change the sewing machine foot to an invisible zipper foot and sew the zip tape in place. You can use a normal zipper foot, but this is a bit tricky because you have to roll the coil back as you sew and you must sew right next to where the coil is attached to the tape.
4. Once the zip fastener is stitched in place on both sides of the fabric, close the fastener. Now, change the invisible zipper foot back to the normal zipper foot and sew the remainder of the seam. This will produce a seam about 2mm below the stitching of the zip fastener. When you have completed the zip insertion you will notice only the zip head.