Insert a pinhook at the back of each pleat at the required height. To enhance the look cord can be looped between the base of the pleats. I used loops of cord twice the width of the spaces and stitched them at the base of each pleat. At each edge the cord was looped to the wrong side and secured in the side seams. To finish off I stitched a covered button at the base of each pleat.
Machine stitch down the full length of the pleat i.e. the width of the buckram.
Remove the tacking stitches. Place the curtain on a flat surface, right side up, and push down the pleat to form 3 equally sized pleats. Hold the pleats in place at the top edge with a clothes peg. Using strong matching thread sew through all thicknesses to form the base of the pleats. Stitch about halfway between the top folds and the line of machine stitches so that the stitches will not show from the front.