First off, why: clear elastic is relatively inexpensive, very flexible, light-weight, and you can sew and cut it (length-wise) and it won't unravel like regular elastic. It gathers evenly and quickly - a nice change for those of us who are 'baste and pull' challenged - and the gathering is done with one line of stitching instead of three (baste two and final secure stitch). Also, it will gather to an exact length (whatever length you cut it to) and takes the ifyness out of elastic thread.
For people with sergers, this stuff is great because you can run it off the edge past the chopper blade without damaging the elastic or disorganizing the gather. Basically, you can use the serger to do the whole gather, no stopping to gather on the regular machine and switching to the serger to finish the seam.
Now, how: Clear elastic comes in a variety of sizes, but I suggest using a 1/2" or 3/8" to start, just to get a feel of it.
Cut the length desired for gather, minus 1/4" (because it stretches out to be slightly larger after sewn.) I usually evenly mark (with pins) both the fabric to be gathered and the strip of clear elastic at 1/4, 1/2, and 3/4 lengths (or whatever makes managable sections) and then pin the two together so the gather will be even. Put your fabric and the end of elastic under the needle, make a couple of stitches to secure, and the stretch it out to evenly match the fabric length. Sew right through elastic.
Keep stretching and stitching until you are done your gathering. Presto, done! You can stitch through the elastic again to attach your gathered fabric to whatever it goes on.
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