Resist the urge to take this one apart, if you can. Just clean the bore and chamber and let it go at that. I don't think the barrel comes off on your model easily without unscrewing it from the receiver, as best I recall.
But if you can't resist... recall it's often easier to take stuff apart than get it back together... and as far as I know the 20 gauge guns are the same mechanically as the 12s, but there are no guarantees here. This is what they call in some places "imperfect information."
CLEAR the magazine and chamber, twice. Three times even.
First step in disassembly is to remove the buttstock. Take off the buttplate or recoil pad, and remove the stock screw with a long shank screwdriver.
YOU MUST REMOVE THE BUTTSTOCK before you try to remove the trigger plate assembly, because the buttstock screw goes into an extension cast into the aluminum trigger plate. Thus the buttstock screw puts pressure on the trigger plate assembly when it is tightened, pulling the trigger plate back against the pin and screw that go through it inside the receiver. That tension must be releived before the trigger plate can be removed.
BE SURE the safety is on and stays on with the trigger plate assy out of the gun. Be sure you don't accidentally detach any of the parts of the trigger plate assy while it's outside the receiver. Things like the slide latch (on the left side looking from the rear) are just sitting there unstaked on a stud, under spring pressure, and waiting for a nudge in the wrong direction to come off. And pulling the trigger with the safety off while the assembly is outside the receiver can sometimes result in an energetic self-disassembly. Did I mention you should be careful with the trigger plate assembly while it was outside the receiver?
To get the bolt assembly out, slide the forearm all the way back till the bolt appears at the back of the receiver (with the stock off and the trigger plate removed, there's nothing in the way). Gently spring the rear of the action bar from its notch in the side of the bolt slide with a thin screwdriver, and remove the bolt and bolt slide to the rear.
On the LEFT side of the bolt slide is the action bar contact lug. This lug is a separate piece on some models and is staked into its notch in the bolt slide. Be sure it isn't loose and doesn't fall out when the bolt slide is removed, if yours is one of these. It can be re-staked with a centerpunch if necessary.
I wouldn't go any further if I were you. You can clean everything that needs cleaning as an assembly without taking it apart any further. There's some weird stuff about removing the forearm assembly that I don't even want to try and describe. The bolt assembly and trigger plate assembly can be soaked in HOT water with some dish soap for a while, rinsed out with more HOT water, and either left to dry from residual heat or gently blown out with compressed air, then re-lubricated carefully.
The inside of the receiver can be cleaned out with a spray of WD-40 and wiped out carefully with paper towels if necessary. Then the places where parts move can be re-lubricated.
The Stevens/Savage pumpsguns from that era are pains in the butt to mess with (compared to more maintenance-friendly designs). And they are somewhat fragile in several places- the cast aluminum trigger plate and the single action bar to name a couple.
If you can clean it well enough to suit you without field stripping it, that's what I would advise...
