Kick pleats, which used to be common in clothing, have today been largely replaced by simple interfaced splits, also known as French vents. Sometimes, however, the French vent is not appropriate for certain workplace situations or formal events. Since the alternative to vented skirts is often uncomfortable tight skirts or flowing casual skirts, a French vent can be altered into a modest vented pleat.
Turn the skirt inside out. Disassemble the hem and stitching around the split, then press the entire back seam toward the right. If your skirt has a lining, take the lining out and perform the same process on the lining as you do the outer skirt. Skirt linings can often be disassembled at the waistband.
Cut two pieces of fabric the length of the split plus 1/2 inch, one with a width of 6 1/2 inches and one with a width of 3-1/2 inches. The fabric should match your skirt as closely as possible or else it is likely to be noticeable. Attach these pieces to the skirt's seam allowances from the bottom up to the top of the split, using the straight stitch on your sewing machine. Make sure the seam allowances of the attachments are 1/4 inch.
Press the new seam allowances flat, then fold back the fabric on the right side so the fold lines up with the original split edge. Press the fold flat. Fold the left side of the fabric back 3 inches, so that the edge of the fabric lines up with the original back seam stitching and the fold lines up with the edge of the other flap. Press the fold flat.
Reinforce the opening at the top of the split with a small piece of fusible interfacing, so that the skirt can hold the weight of the kick pleat. Continue the back seam stitching over the piece of interfacing, then rotate the fabric 90 degrees and stitch straight across to the edges of the flaps, joining them.
Fold up and stitch the hem back into place, including the new kick pleat flaps. If your skirt had a lining, attach the lining to the flaps and hemline and reattach the lining to the waistband.