Installing a trapdoor through a plywood subfloor is a simple procedure requiring less than an hour in most cases. Framing needs to be added underneath to support the door; then it is just a matter of cutting the trap in the plywood and adding hinges to make the door easily accessible. Remove any carpet from the area before starting installation.
Choose a location for your trapdoor. Use an electronic stud finder to locate the floor joists on either side of the door. Depress the buttons on both sides of the device and drag it along the floor, marking each spot where the finder beeps and lights up. Do this in a line 6 inches on either side of the trapdoor, about 36 inches long. Connect the marks to outline where the joists are located.
Draw a 24-inch square for the trapdoor on your floor, with two of its edges centered on joists, which are spaced 24 inches center to center. Cut the trapdoor out with a circular saw with the depth set to the thickness of the floor, typically 3/4 inch. Lift the plywood square from the opening.
Cut two pieces of 2-by-4 to fit between the two joists supporting the edges of the trapdoor. Screw them between the joists with 3-inch treated deck screws so that the center of the top edge of each piece is aligned with the edge of the cut opening, providing a 3/4-inch lip around the opening on all sides. Lay the plywood on its top, with one edge aligned with one edge of the hole. Screw two 1-inch utility hinges to the edge of the trapdoor and the edge of the floor.
Prop the trapdoor up and use a 1-inch paddle bit to bore a hole, centered, 2 inches from the edge opposite the hinges. Close the trapdoor back into its original position.