How to make a set of retaining garden steps

The second step riser and the sides

4) Prepare the ground behind the tread

Dig back at least 100mm (4″) behind the tread.

Dig, pack, and/or fill and tamp

5) Position the next riser

Place the next riser on top of the firm and level ground directly behind the tread.

6) Cut and fit the side boards 

Cut the side boards out of 150mm x 50mm (2″ x 6″) treated lumber.

Measure, mark and cut the first board on each side so that the top of each board is level with the top of the uppermost riser.
Then cut the ends of the side boards flush with the retaining wall post as shown in the picture.

Next measure, mark and cut another piece of 150mm x 50mm (2″ x 6″) treated lumber to fill the gap between the bottom of the previously fitted side board and the ground.