Thanks very much for all your feedback on my post regarding the design lines for my upcoming sheath dress – I decided to go with the top design, and save the bottom one for some later colour blocking (maybe in ponte jersey?).
Once that was decided, I sewed up that basic Burda sheath dress (which fit me very well, as expected), then while it was on my dressform I drew rough design lines on to match. After the rough lines were on, I cleaned these up with French curves while the dress was lying flat (these photoshopped ones are just free-handed on top since my black lines on navy blue fabric were hard to see in the photo!)
I then cut apart the Burda muslin along my new lines, and cut into the curves a few times to release any bumps. I then transferred these altered pieces onto a second muslin to test that all my new curves matched up well: