Remodeling a wall to install a new opening requires a bit of "demo" work and removing plaster, drywall and framing will kick up a lot of dust. To minimize clean-up, cover all furniture and tape plastic over all openings to contain the dust while remodeling. Also, wear a dust mask and safety glasses when breaking plaster off walls to protect yourself from the debris.
Choose the location of the new door or window and determine if any plumbing or wiring will be affected by the new installation. Look in the attic or basement to locate pipes and wires. If no access is available to these spaces, use a hammer to knock small holes in the wall to find plumbing and electrical systems.
If plumbing pipes are discovered to be in the space for the new opening, try to find another location that will do as well, but doesn't require relocating plumbing. Moving other obstacles will always be easier than moving plumbing. It will be easier, for instance, to relocate an electrical box than a water pipe.
Begin the wall demolition by removing the baseboard. You'll need to drive a wide blade behind the molding to pry it off without damaging the wood. Start by running a sharp drywall knife along the top of the profile where it meets the wall to cut any caulk that may be sealing it to the drywall. Next use a wide chisel-bladed joint knife wedged behind the molding to begin to loosen it. Drive the blade behind the baseboard at each wall stud and pull out on the handle a few times to begin loosening the nails. When the gap is wide enough, hold a piece of plywood against the wall to protect it and slip a claw hammer or crowbar behind the molding to pry it completely off.
With the moldings removed, locate an existing wall stud that will can serve as one of the king studs for the new frame. Check out this article for help making the choice and use the measurements from the framing instructions to mark the wall for the new opening.
From the inside of the chosen stud, measure along the floor to the location where the second king stud will be and mark the wall there. Use a level to draw a plumb line from the mark, up the wall to the location of the new header. Use the level to draw a line to mark the inside of the first king stud and then to join the two sides at the top marking the header location.
To remove drywall first cut along the lines with a reciprocating saw or a drywall saw in the hollow parts between the studs. Use the shortest possible blade on the reciprocating saw and hold it at a slight upward angle while cutting to minimize the chances of hitting the opposite wall as it moves in and out.
Knock a hole in the drywall with a hammer to insert the saw blade. Push it all the way in and press the base firmly against the wall surface. Keep firm pressure against the drywall while operating the saw to avoid vibrations that can damage the wall or cause injury.
After the outline is cut out along the hollow parts, cut the drywall over the studs by scoring several times with a sharp drywall knife until it's cut completely through. Next, break the drywall out of the opening by hand. Use the side of the hammer to make an opening large enough to get a grip and pull the drywall off in pieces. Bag the pieces and then finish by removing any nails or screws from the exposed studs.
Plaster is very brittle so to minimize damage to the surrounding wall tape along the outer edge of the outline with masking tape. Next, score the outline using a drywall knife or the side of a sharp chisel. Score repeatedly and deeply (an eighth inch or more) to encourage the plaster to break cleanly along the line.
Use a 2x4 and hammer to tap the wall just inside the outline, breaking the plaster along the scoring. Use a chisel-blade and hammer to cut any plaster that doesn't break along the line. Then use a sledge hammer or the side of a claw hammer to break the plaster brown and putty coats free from the wood lath in the center of the opening.
With all the plaster removed, use a reciprocating saw to cut the wood lath strips along the outline of the opening. To avoid vibrations that will crack the surrounding plaster keep the base of the saw tight against the surface as you cut. After all the pieces are cut, pry the wood free from the framing using a crowbar and claw hammer. Remove all remaining nails and screws from the 2x4 framing. You're now ready to cut the studs for the new door or window installation.