In a fast nut shell.
Use a torch to burn the rubber between the lock ring and the cover.
Us light hammer to do lots of light hits on the cover and lock ring. The goal is to break the crap free and slightly expand the cover (You shrink to fit with heat later).
Use nice strap tool you make to get cover loose.
You now have to get the inner cover off. You will need a large deep impact socket.
If there is rust on the inner cover then heat the base some and quench a few times (it is unlikely the shock is good, but you never know).
Use 3/4" air impact to break the cover loose.
Use heat and quenching to get the valve out of the vane shaft. In a junk shock the valve could be important for another shock.
Evaluate shock.
Rust in the cavity, no good.
Loose and worn, no good.
A bit loose, a few thou wear, it may work with much thicker oil.
Tight, bingo, but may have enough wear that somewhat thicker oil is needed to get close to spec of rate of movement.
Obviously, there are a lot of details left out. Including the building of some tools to take off the cover and the locking ring.
I do not use penetrating fluid, it is a waste of time here.
