Scallops are formed by stitching curved seam lines when you attach a facing (or back piece of a collar). In the image above you'll see the two facing options that came with my pattern - the one on the left is if you want a straight jacket front and the one on the right is if you want a scalloped jacket front. So it's pretty simple to convert your pattern piece a scalloped one - just draw the curved lines onto it (or straight onto the fabric if you're feeling confident). You could use a tin lid or something else with a curve as a template. Don't forget to leave a seam allowance around the curve. You may need to create extra seam allowances to accommodate your scalloped edges, as the inner corners of the scallops are going to be further in than the outer edge - for example, if you don't want to make your skirt too short!
Pin the facing and garment piece right sides together.
To sew them together, set your machine to a very small stitch, sewing slowwwly and smoooothly along the curved lines. I'd recommend practising on a scrap first. It helps if you use fabric that isn't going to slide around on your machine. Once you've sewn all along the line, stitch back over the inner corners of the scallops to reinforce them.
Next, trim your seam allowance down to about 1/4", then clip the curves, being careful to cut as close as you can to the stitching without cutting through the thread (as I did - oops!).
Now comes the ironing stage, which took me the best part of an afternoon (I'm not kidding). With right sides still together, press between the layers, running the point of your iron right up and along the stitching line. Then turn the layers right sides out, and do the same thing with the point of the iron, running along the inside of the scallops.
Then press from the top through both layers to smooth down the line. I discovered that a good way of making the curve more defined is to run a butter knife along the inside of the curve while pressing. It's fiddly, but worth the effort to make your curves more curvy. It helps to work on one scallop at a time, coaxing the curve using a triple-pronged attack while the fabric is malleable, using the point of the iron inside, then pressing from the outside while running the knife along the inside of the curve.
Et voilà! Curvaceous scallops. Excuse the unfinished-ness of the rest of the jacket - sleeves up next.
Hope you found this useful. This was of course just what worked for me. If you've tried making scallops and have your own tips, do share!
[Soundtrack: 'Heartbreaker' by The Crystals]