You will need...
NB: The top or garment needs to made from synthetic fabric or it can't be moulded; natural fabrics, such as cotton, have too high a melting point and don't behave in a heat-malleable way.
Pins need to be as narrow as possible; ideally, you should avoid pins with round or wide heads.
Quite easyI fear I might be jumping the gun in assuming that it's a complete doddle, since there is a smattering of technique involved. It helps to be good with your hands and especially dexterous with your fingers. Confused? All will be revealed.
TimeThis wasn't as quick as I expected - I'm not entirely sure if that was down to perfectionism or being distracted by the television, which I like to have on in the background - but if you do the design I did it shouldn't take more than a couple of hours.
Concertina-fold the fabric , lengthways, along the middle of the top, or wherever else you want your smocking to be. Pin it in place and iron the pleats on as high a setting as possible - your aim is to get the plastic to change shape through heat without melting it. I would recommend working up to the highest temperature you can and practising on some scrap fabric, if possible. You can also heat-set some synthetic fabrics by boiling them in water for a short while - sorry I couldn't be more helpful with timings but it's been a while since I last attempted that particular method!
After you have fixed the pleats, unpin them and pin the tips of the pleats together at alternating intervals to make diamond patterns. Hand-stitch or glue the tips together and unpin them. Since I was doing my design lengthways, I didn't do diamond shapes completely along the pleats, ; I stopped about a third of the way down, and would recommend you do the same, so that it fits more easily.