The loose felt and glue that is left behind needs to be removed. A heat gun and scraper works great here (Image 1).
Note: Don't overheat the tile, you just want it to bubble — just loosen the glue.
A "slam scraper" is also called a "stand-up scraper" (Image 2) and rents for $15 a day. Be careful because it is a sharp tool.
It is a good idea to rent a dumpster when you are doing a remodeling project. You can rent a 6-yard dumpster for under $200.
Dry fit the cement board before you lay down the thin set and screw it in place.
If any wood on the floor looks stained (this is common around the tub area — usually just a little delamination), you can check it with a screwdriver. If the screwdriver does not go through the wood, it is okay. If any wood is rotted out, you would need to replace the wood before you put down the cement board
After taking careful measurements around the room, cut the 1/2" thick cement board with a circular saw with a carbide blade. Work slowly, don't force the saw through the material, and let the blade do the work.
Note: The cement board must be fully supported, so it doesn't break as you finish your cut.
To cut holes or make notches, use a jig saw with a carbide blade. Again, work slowly and let the blade do the work.
After you have all your boards cut, lay them loosely on the floor first to make sure they fit before you glue them down with thin set.
Use a heavy-duty, low speed drill with a mixing paddle to prepare the thin-set. Mix it until it is creamy.
Note: You can rent a drill for approximately $26 a day.
Apply the thin set with a 1/4" x 1/4" square notched trowel. This provides a heavy application that will give you good adhesion.
Start laying down the cement board, working your way out of the room.
Note: Cement board is always used under ceramic tile because it is stable, even when it is exposed to moisture in a bathroom.
Fasten the cement boards with 1-1/2" galvanized deck screws. Drive them in about every 6" to 8".