Don't worry, it always looks bad but at least you can see the bottom of the internal jacking bracket, so with a bit of luck, it'll only be the outer sill skin that needs replacing
Just make sure you clean off all the surface rust internally once the outer section is cut off, treat with rust converter if required, prime, gloss then eventually spray internally with waxoyl or similar when everything is painted & finished.
If you can find a donor car with good sills, cut the sections out of that - makes it much easier to repair and a good, finished job
