Dry Fit the Area
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.