Measure the length and width of the room and multiply the two together to determine the total square footage. Divide that number by the square footage of a sheet of plywood subfloor which is 32 to get the number of sheets that will be needed for installation.
Line up the first piece of plywood vertically beginning at any corner of the room. Place the first 3/4-inch ring shank nail at the bottom corner of the first sheet leaving 3/8 inch from the edge of the panel. Hammer it in place and move 2 inches down the plywood. Repeat the process for the entire sheet. Insert a nail 2 inches around the entire perimeter.
Snap a chalk line diagonally from the corner where the plywood has been installed to the opposite corner. Hammer in nails diagonally along the chalk line every 4 inches into the plywood. Fasten the rest of the panel at 4-inch intervals around the entire sheet to secure it to the sheathing underneath.
Butt the second sheet of plywood subfloor directly into the first. Add nails every 2 inches around the perimeter and every 4 inches along the inside. Continue around the room nailing the plywood to the sheathing underneath until reaching the end of the first side.
Measure and mark the plywood with a pencil where the cut will need to be made. Cut straight along the line with a circular saw and a fine-toothed blade. Line up the piece and install it just like the previous pieces were installed. Move around the entire room until all of the 1/4-inch plywood subfloor has been installed.