Lettuce edges are those that appear to have a fluffy somewhat rigid ruffle, and are an easy way of adding some interest to an edge that would otherwise be boring. I’ve seen it mostly used for chiffon skirts, although I’ve used them for sleeves, and have even seen them in veils and even pants. They are very easy, if a tad laborious, to make. You MUST have an overlocker (serger) to make them, with care and patience. You will also need nylon fishing thread, or the kind used for jewellery making. It must not be stretchy, and needs to have a certain rigidity. The fabric you are using will work better if it has a certain give at least, as this will create even more of a wave.
Start by setting your overlocker to do a rolled hem. Loose a few coils from the fishing thread bobbin, then very carefully roll the edge of the fabric over the thread, and pull the thread across… you will be better to have quite a bit of thread extra as it will help you keep everything in place.
Put the rolled fabric onto the overlocker, lower the foot to lock it in place, and start sewing a rolled hem SLOWLY while stretching it very carefully. Be careful to keep rolling the hem by hand, as it will give you a neater edge, and avoid sharp angles and corners, opting instead for soft curves. Once you’ve reached the end, finish as usual; you can usually turn the remaining thread back and into the machine-rolled hem, or over the start if you’re doing a circular edge.
It is highly advisable that you practice this on cheap fabric or scraps, to get the hang of it. You will get the feel for it eventually, after sewing a couple of metres, but in the meantime you are likely to accidentally cut the thread or make a booboo at least a couple of times. Don’t give up, as this technique will come very useful, and it’s worth the time it takes to learn to do it. The stretchier the fabric, the more you exploit this stretch, and the more rigid the thread, the better the frill you will get.