I know this elementary to most but a few may need the knowledge and most might learn one more tchnique
1] Soak in penetrating oil (Liquid wrench, Marvel Mystery Oil, diesel fuel, WD40, oil of wintergreen, bore cleaner). Allow time to work. I like to soak the rusted fastiner once weekly for a month or two with diesel fuel.
2] gently hammer the proper sized bolt out from Sears, Irwin, Home Depot and twist counter clockwise
3] if broken off then stud remover (if stub is long enough) or spiral extractor, or multispined extractor after drilling proper sized hole.
4] heating with torch then applying penetrating oil when warm not red hot
5] burning the nut off with torch
6] using torch to weld another nut
7] striking the bolt or nut with hammer and with medium force then more
8] cutting groove in nut with dremel and then enlarging groove with chisel.
and then more penetrating oil.
9] cutting off nut with cut off wheel.
Si hoc legere scis,nimium eruditionis habes.