Set a sheet of Hardiboard on one corner of the floor. Trace around the perimeter of the board with a pencil. Take up the board.
Spread a layer of thinset mortar onto the marked-out area with a mortar trowel, covering the area completely. Set the Hardiboard back in place, pressing it down fully into the mortar. Walk around the board to ensure that it’s fully secured.
Repeat the process for the next board, setting it at the end of the first board, tracing around it, spreading down mortar, and setting the board in place.
Lay the boards end to end along the wall. Cut the final board in the course with your jigsaw so it fits against the side wall. Start the second course alongside the first but at the opposite end as you started before, so the seams are staggered.
Cover the whole floor with Hardiboard, course by course. Let it set for a day.
Press drywall tape on the seams between the boards, covering all of them. Spread thinset mortar over the tape lines, using a drywall knife. Get the mortar smooth and even. Let it set overnight.