Unfortunately, I've researched those methods AFTER I cut up a pair of 1930's french knickers.
This is what it looks like now.
It had lovely details at the bottom
The material (silk) was worn out and literally threadbare at the crotch
and it looked moth eaten too in places
but, still, I regret cutting it in scraps
Second, the technique used for the crotch is called "incrustation" in french, I don't know its name in English and didn't even imagine an angle could be sewn that way. I have to practice that technique and then, I might be able to redo this without the freaky crotch.
so, this is what I learned : research and document myself before starting a project and avoid destroying an antique.
Here is how to sew an "incrustation" :