Keeping clothing looking fresh and crisp would be much more simple if everything we wore was flat. But most of us like a little shape to our clothes and pleats, ruffles and ruching add some interesting and flattering detail.
Pleats can be heat-set or stitched in place. Heat-set pleats hold their shape thanks to the type of fabric selected a high heat process. Accordion pleats are most often heat-set and are a softer, less crisp pleat. Dry cleaning will best preserve heat-set pleats. Stitched pleats are usually sharper and have less give because of the stitching. They will hold their shape when laundered on a gentle cycle if the fabric is washable.
Ruffles are fabric that has been softly gathered and then stitched into place on a garment along the gathered edge. The folds are usually soft and loose and seldom stitched into place.
Ruching, or draping, are soft folds in a garment that are only secured along the seam edges. The shape is fluid and does not need sharp edges.
For a stitched knife-pleated skirt, use a full-size ironing board. Attach a paper clip, binder clip or spring-loaded clothespin to each pleat's fold to keep it in place. Using a pressing cloth between the iron and the skirt, place the iron at the top of the pleat and press at the correct temperature for the fabric. Pick up the iron and set it down again farther down the pleat. Never drag the iron across the fabric. Pleats can pucker and shift. Never iron over the clips because it will leave a mark. Carefully work your way around the skirt.
Iron pleated shirts, like tuxedo shirts, from the wrong side first. Then turn the shirt right-side out and press a small area at a time. Repeat the same process as the pleated skirt. You will probably not need to use clips to hold the pleats in place unless they are very large.
For ruching and soft folds, use a steamer to avoid sharp creases. Hang the garment and carefully move the steamer over the fabric. If you don't have a steamer, filling a bathroom with steam and hanging the garment on a shower rod will often release any wrinkles. Allow the garment to dry thoroughly before wearing.
Specific questions? Just ask here.