You can buy a prefab access panel to make a neat opening in walls and ceilings for reaching plumbing or other household systems. These panels are easy to install and come in sizes from about 10 inches to two feet or more.
Check the packaging for dimensions to cut an opening that will accommodate the panel. Start by setting a level against the wall to draw a straight, level line for the top. The opening will be about an inch below the edge of the frame, so plan the placement to accommodate that.
Next, measure from the top line, down to the bottom and place a pencil mark corresponding to each end of the line. Set the level or other straight-edge on the marks and draw a line to mark the bottom of the opening.
Next, set a square against the top line to form a right angle and draw a line to join the top and bottom, marking one side of the opening. Measure from there to the other side, at top and bottom, and place a mark for the other side of the opening. Set the straight-edge on the marks and draw a line for the other side.
On drywall cut along the top line and down the two sides with the saw and then score the bottom line with a drywall knife. Cut deeply and then push in on the drywall to snap it along the scored line. Cut through the remaining paper on the inside of the square to free the drywall cut-out and let it fall. You can leave it inside the wall or pull it out after it's free.
To cut out wood-lath plaster, drill a hole just inside the corners of the square and use a reciprocating saw to cut between them, along all four lines. Be careful to keep the saw pressed firmly against the plaster while cutting to avoid causing cracks and other damage.
Remove the panel cover using a pen knife or screwdriver to pry at one of the slots. Apply a bead of acrylic latex caulk along the back of the frame, going all the way around to leave about ¼ bead.
Set the frame into the square hole and press firmly to spread the caulk, sticking it to the wall. Mark sure the frame is straight and push hard all the way around the perimeter. Clean off any caulk that squeezes onto the wall using a wet cloth and check for gaps. Let the caulk set for a couple of hours before reinstalling the cover.