Inspect the Durock boards already installed on the floor and verify they are secure to the sheathing or subflooring underneath. Verify all screw heads are flush with the Durock and the gaps between the boards are not more than 1/8 inch.
Place self-adhesive Durock cement board tape over the seams between the pieces. Start at one end of the room and roll the tape across to the other over all of the seams. Cut off the tape at the ends with scissors and continue in the same manner until the tape covers all of seams.
Run your hands over the tape and push down firmly so it can adhere to the Durock. Open up a bucket of thinset mortar and stir it up with a notched trowel. Dip the notched trowel into the mortar and spread it out over the tape covering the seams.
Spread out a thin layer about 1/8-inch over all of the seams. Do not worry about getting it all over the Durock because it will be covered with tile or other materials. Dip the notched trowel back into the mortar when necessary and continue spreading out the mortar until all the seams are covered.