Before installing a tile floor, a subfloor and underlayment is necessary.

Mark the Cut Lines

Use a T-square to mark the cut lines on the plywood.

Cut the Plywood

Cut the plywood to size, running the circular saw across the width of the board.

Mark the Joist Locations

Using the original sub-floor nails as a guide, mark the floor joist locations on the wall to know where to insert the screws (I personally snap a chalk line)

Lay the Plywood

Lay the plywood in place on the floor.

Insert the Deck Screws

Run a chalk line off the floor joist to mark the wall. Then, drive deck screws every 8 inches along the line.