What is the preferred tool for removing the plug? Well fitted adjustable wrench?
That will work, you may need a length of pipe to provide more leverage.
First though, why do this? You are doing it to assure yourself that the fellow who fitted the plug did a proper job, a breech plug that isn't properly mated will allow super hot gases to escape past the plug face, cut the threads and cause corrosive powder residue to degrade the plug over time - thus creating unsafe condition. I, like the majority here, check all plugs, always. Once I've fired a few hundred shots in a gun I remove plugs and check them once again.
The plug will be tightly fitted, as it should be, you may need to use a mallet to persuade the plug to break free and unscrew (counter clockwise) . You will want to clamp the breech end of the barrel firmly in a machinist style vice, use something to protect the barrel from the vice jaws - I make the protectors out of flattened copper pipe, use tape to hold the copper in the vice. You might also want bend a length of flattened copper tubing on the breech plug, bend it in a U shape. Keep the wrench jaws below the top surface of the plug so you don't mar it. In case you are interested; Jason at Rice barrels sells a plug wrench and octagon shaped barrel protectors that work wonderfully.
Once out check for (god forbid) cross-threading or any galling on the threads, they should look factory new. It is important that the face of the breech plug mates to the inner "ledge" or shoulder of the barrel (there is a name for that but it escapes me at the moment - you'll see it). For this I use Dykem Hy-Spot Blue (get a small tube, although it is more expensive than a can - the tube will last a very long time if stored cap down). You don't want the Dykem "layout fluid" for this job. Permatex Prussian Blue is sold for this purpose also but I find that in use the Dykem is easier to see. You will also want to coat the breech plug threads with thin coat of antiseize compound before reinstalling it, it does not take very much compound and be sure to wipe any excess off the outer surface of the plug and barrel.
Using a small inexpensive brush to paint a
thin coat of the blue around the outer diameter of the plug face and run it back in until it is firm but not fully seated to the witness mark, unscrew it and check that the blue gunk transferred to the inner surface of the barrel evenly (it is easier to see on the plug face). Satisfy yourself that the two surfaces mate. Cleanup is with WD-40.
This is a good time to polish the face of the breech plug, doing so makes future cleanup easier after shooting as the powder residue will remove more readily form a polished surface. I polish mine to a mirror finish. For inletting send the barrel off without the plug installed. When you receive the barrel and stock back you'll want to draw file the barrel and take it to whatever degree of finish you intend.
Hope that helps, If I've missed anything surely someone will chime in.
dave