Vinyl flooring doesn’t need to have an underlayment cement board, as is the standard for hard tiles, but it is a good idea to have a smooth, solid base under it. Plywood is an acceptable underlayment for vinyl, and can be installed directly over the subfloor, or over a regular wooden floor. If you’re installing over a subfloor, make sure that the direction of the plywood underlayment sheets is perpendicular to the direction of the plywood sheets that make up the subfloor.
Let the plywood sit in the room where it’s going to be installed for a week before installation, to allow it to acclimate to the environment.
Clean the subfloor and let it dry completely. Set the first sheet of plywood on the subfloor in one corner of the room. Leave about ¼ inch of space between the edges of the board and the two walls, to allow for the plywood to expand if necessary.
Drive wood screws straight down into the surface of the plywood with your screwgun, putting them every 6 inches around the edges of the board (about an inch from the edge), and then every 10 or 12 inches across the middle.
Set the second piece of plywood at the end of the first one, alongside the wall and sitting ¼ inch from it. Butt the edges of the plywood tightly to each other.
Proceed to lay the rest of the course, end to end, to the far wall. Cut the last board with a circular saw to fit, using your t-square to mark a straight line across the width of it.
Start the next course at the side of the floor where you ended the first course, so the ends of the boards are staggered and there are no four-way intersections between the boards. Repeat and cover the whole floor, pressing the edges of the boards tightly to each other while leaving ¼ inch all around the perimeter by the walls. The floor is now ready for vinyl.
Wear eye protection when using your circular saw.