Clean the pressure treated wood subfloor with a broom and dust pan. Cacuum over it to get up all dust and dirt.
Squirt an adhesive over the pressure treated plywood and lay down sheets of polyethylene to create a vapor barrier. Allow it to dry overnight or as long as suggested by the manufacturer.
Fasten backer board over the polyethylene vapor barrier with stainless steel screws using a screw gun.
Spread thin-set mortar over a small area in the center of the room with a trowel and lay the first tile in the mortar. Place four spacers around each corner of the first tile. Spread more thin-set mortar and set the second tile, along with spacers.
Continue to lay tiles in thin-set mortar, accompanied with spacers on the corners of each tile, going from the center of the room out until the entire pressure treated plywood subfloor is tiled over. Let the tiles sit overnight.
Take up all the spacers and spread grout between the tiles on the pressure treated plywood subfloor with a trowel. Immediately wipe up excess grout with damp cloths. Allow the grout to sit overnight before using the tiled floor.