Fasten the backer board to the floor using proper nails or screws, as recommended by the manufacturer, every 8" around the perimeter and all supporting the studs. Keep fasteners between 3/8" and 3/4" from sheet edges and 2" in from sheet corners. Make sure that the nail or screw heads are not sticking up above the board surface.
Note: In some cases, you can attach the backer board joints using 2" wide glass fiber mesh cementitious alkali-resistant backer unit reinforcing tape (see product instructions – usually needed for wide areas). The tape will help disperse any movement of the boards and decrease the probability of popping or cracking tiles along the seams.