Before you lay new wood flooring, vinyl tile, carpet or other floor materials, you'll want a smooth, even sub-flooring installed over the old floor. This provides a solid, level work surface to affix the flooring and adds stability to prevent the floor from creaking or moving. For most flooring materials other than ceramic tile, a plywood sub-flooring will work just fine. (If you're laying ceramic tile, then plywood sub-flooring isn't recommended, because a more rigid surface is needed to prevent the grout from cracking. For ceramic tile, sub-floor with cement-board instead of plywood, and otherwise follow the same instructions.)
Things You'll Need
Enough 1/2- or 5/8-inch thick 8 by 4-foot sheets of plywood to cover the floor
Carpenter's glue with glue gun (see container for square footage covered by each tube of glue)
Power drill with screwdriver bit
Box of 1-1/4-inch drywall screws
Lay the first 8 by 4 plywood sheet, top side down, along the wall in one corner, so two sides of it are bordered by wall. Use your glue gun to spread the glue over the sheet, in a criss-cross pattern that comes within a few inches of the outer edge of the board of the sheet and doesn't leave more than one square foot of plywood without glue on it.
Flip the sheet over and press it into place where it was, with the glue side down. Using your drill with the screwdriver bit, sink a drywall screw every 6 inches around the perimeter of the piece, and about every 1 foot in the rest of the piece.
Measure the space left in the room from the short end of the plywood to the far wall, putting your measuring tape at three different spots along the edge and writing each measurement on the plywood. If the space left is less than 8 feet, transfer all three measurements to a second sheet of plywood, line up your level along all three marks, and draw a straight line between them. Set the piece up on your sawhorses and cut with your circular saw. (If there is more than 8 feet from the edge of the sheet to the wall, put another full sheet there, securing it in the same way, and then measure and cut the next piece.)
Lay the cut piece in the space between the previous sheet and the wall, with the cut side toward the wall, to make sure it fits. If it does, remove it, apply the glue directly to the floor in the same pattern as you used previously, lay the piece there, and screw it down as before.
Lay the next course of plywood sheets alongside the first course, creating a tight seam at the 8-foot sides and staggering them so that you don't have any four-way intersections from the 4-foot sides. Generally, you can achieve this by simply starting the whole process from the other end of the room, unless it happens that all the sheets in the first course fit evenly with no cuts. In that case, use your circular saw to trim a foot off one end of the first sheet before you lay in the next course, so the short seams don't line up.
Measure and cut the final course to fit, cutting the sheets lengthwise if necessary, and glue and screw them down.
Always wear safety goggles when using your circular saw.