One of our people will act as St. Wenceslas at a festival in Prague this week. Who will make the costume? :-)
I prepared a pattern from another piece of fight clothes. I usually take only the pattern for the neckline, armholes and the upper part of the sleeve.
I sewed an oval to where the slit would be.
I cut it in the middle, turned the bands in and basted, leaving a mm or two from the bands visible. This will cause that the slit won't have the usual compulsory distance between the right and left half. Unfortunately, it should have been facing the right side out, but I didn't manage to:-/
Before I sewed it on machine, I basted the end of the slit so that it wouldn't open. Then I secured this with a line of stitches near the edge.
I made this costume according to a picture of St. Wenceslas, he had a wavy lower hem. I let the seam allowance as small as possible so that I could easily iron it before sewing. When ironing, I was using a paper pattern of a wave, around which I was folding the hem.