Hi, Becky! Yes, you'll cut the placket 2x the length of the slit, but it will stretch a bit when you insert it, so you may have a bit to trim off when you reach the end. That's perfectly fine and no reason to worry. Have fun sewing!
Okay, I already knew I wanted one (or two!) when I got the newsletter, but now you have me positively drooling. Thanks, Becca!
Works great! Because curved seams are on the bias, they will naturally "bend" as you go. Just remember that you are sewing very close to the original seam. If you try to stitch the facing/lining too far out, it will pucker around the curve.