My grandma taught me to knit when I was wee. When she turned 90 last year, I decided that instead of buying her a present, I would knit her something gorgeous. It seemed like a nice, circular thing to do, and an impressive enough gift for such a momentous birthday.
I followed a pattern, but actually, you could do this with any lace pattern that has a scalloped edge. You cast on and knit it up to half the length you require, put those stitches onto a holder, then cast on and knit the other half. Join the two halves using Kitchener stitch, which is not so much a seam as an invisible join. That way, you get to have the scalloped edge at both ends.
Choose a lace pattern that you like. These normally use a certain number of stitches for each repeat, plus a few stitches. Knit a small stitch gauge, to see how wide each repeat will be. Work out how wide you want the shawl to be, and divide that by the width of a single repeat to find the number of times you will be repeating the pattern. (Round it up or down to the nearest whole number.) I added several repeats to my original pattern, because when I started, it looked a bit mean: more like a scarf width than a good, generous shawl.