Mercedes 123 pleats Deep pleats are achieved by cutting a thin channel in the sew foam. You need to make the width of the material wider to allow extra material to flow into and out of the channel. If you are using 1 inch foam and the pleat is 1 1/2 inches wide, the material would need to be marked at least 3 inches wide. It would be best to cut a strip of material and lay it in the channel to determine an exact measurement.
To mark the foam, lay out your pleats and use a sharpie to draw the lines on the foam. With a new razor blade cut along the outside of the line on both sides of the mark to give you the thin channel. Be careful that you do not cut all the way through the foam backing. Remove the thin piece of foam.
Start sewing from the center of the insert and work your way to the outside. Chalk the sew line and use it as a guide to keep the stitch straight.
A narrow presser foot will also help when sewing.