Plywood is a common subfloor material and is most often the surface that tiles need to be installed over. While plywood is for the most part flat and level, to ensure long-lasting tiling, prime and prepare the plywood before installing the tiles.
Mix up the thinset mortar following the manufacturer's instructions. It should be a peanut butter-like consistency.
Spread the thinset onto the plywood subfloor in an area the size of one of the backerboard sheets. The thinset should be around 1/8-inch thick.
Press the backerboard into the mortar, then reapply mortar to the rest of the floor, laying the backerboard down as you go. Stagger the joints of the backerboard to increase the integrity of the floor. Use the utility knife to resize any pieces of backerboard that are too large to fit on the plywood subfloor.
Check the floor with the level to ensure that an even plane is achieved. If any backerboard is sitting low, apply a small amount of more thinset under it to bring it to level. Once you are happy with the floor level, you can secure the backerboard to the plywood subfloor.
Drive backerboard screws around the perimeter of each backerboard piece every 6 inches or so. Screw down every 6 inches in the centre of the backerboard pieces as well.
While tiling can be done directly onto plywood subfloors, it increases the strength and durability of the tile work to prime the plywood with mortar and a backerboard underlay first.