See my welding thread, have the same issue, although yours is much worse than mine.

Like you I have never touched a welder, but it's a skill I want to learn.

Replacing an inner sill is a big task, as the sills are where the strength in the car lies. I think all inner sills are flat plates, however when formed with the outer sill, it provides a rigid structure.

So far as I am aware, you need to brace the shell and make sure things like doors are in alignment before you weld the new one on.

This is what I had in mind, but in my case I am hoping I can get away with repairing.

However in your case I don't think you have any chance of good metal to weld to.

Fortunately, new sills are available.

Martin