Step 1 - Applying Thinset
Before you actually attach the cement board to the floor, you will first need to apply a layer of thinset to the floor. Many people end up skipping this step and they run into problems with the cement board moving. If you want to make sure that the cement board has a level and supporting surface underneath it, applying thinset is going to be your best bet. Use a v-notched trowel to apply the thinset to the surface of the wood subfloor. Make sure that you apply this thinset evenly and smoothly to the floor. You do not want one portion of the thinset to be much thicker than another. You will then need to firmly press the cement board down into the thinset before moving forward.
Step 2 - Nailing the Cement Board
Once you have the cement board down onto the floor and pressed into the thinset, you will need to attach it to the wood subfloor below. There are a few different ways that you can do this. One method that you can use is to nail it to the floor. For many people, this is the easiest and most effective way to secure the boards. If you are going to nail the cement board down to the wood subfloor, you will want to make sure that you get the right kind of nails for the job. You want to make sure that the nails are long enough to go through the cement board and down into the wood subfloor. You should also consider using hot-dipped ring-shanked nails for this application. They tend to work better and hold up well. When nailing the cement board to the floor, many people have the tendency to not install as many nails as they should. Fill each one of the holes on the cement board with a nail. This will make sure that the cement board stays in place and does not come up.
Step 3 - Screw the Cement Board Down
Besides nailing, you also have another option when applying the cement board to the floor. You can also screw it down. Make sure that you get screws that are long enough for the job and then use an electric drill to screw them in place. Again, you want to put a screw in each one of the holes on the cement board.
Selected from: https://www.doityourself.com/stry/what-is-fireboard