Shut off the power circuit for the area, cover all electrical plates with plastic and put plastic sheeting, or a drop cloth, down on the floor to protect it. Measure out the area on the wall where you want to create the opening. You can mark it with a pencil or chalk line. Try to choose a location between two wall studs for easy framing. Use a stud finder to locate the studs.
Use a utility knife or hand saw to cut the drywall and remove the portion of the wall without damaging the remaining parts of the wall. You might need a small hammer or crowbar to remove smaller bits that are stuck to the frame.
Move any wires that you find in the wall to one of the areas that you plan to leave intact. You can just use your hand to push the wires up, down or to the side.
Use a reciprocating saw to cut through the center of the wall studs within the area where you want to create the opening. You can pull on the top of the stud and give it a twist to remove it.
Stack two 1-inch thick boards on top of each other along the top of the opening you created. Secure them in place with evenly spaced nails. Repeat with a single board across the bottom of the opening. These serve as your header and footer in the opening.
Nail a single 1-inch thick board on each side of the header and footer. Once you do this, you have framed the opening, which keeps the structural stability of the load-bearing wall intact.
Apply drywall plaster via a drywall knife to any areas of the remaining drywall that you damaged in the process. Once it dries, you can use a sanding block to get it smooth, then repaint, if necessary.