If your not sure what that means its simply using a discardable material inside the box to make the sidewalls the desired thickness.
How to form concrete steps with side walls.
Mix the concrete and bring it to the site in wheelbarrow loads.
Then pour your concrete inside the form using a chute or hose attached to the mixer.
Before pouring concrete steps coat the forms with a release agent.
You would cut a form for the inside that would be similar to a stair stringer but set upside down.
To make the wall as strong as possible pour the concrete in layers no higher than 20 inches working from 1 end of the wall to the other.
Use screws and a power drill to attach each riser form board to your stringers.
If steps are longer than 4 feet install stringers to support risers and keep stair lines straight.
Assemble your concrete form.
Shovel the concrete inside the forms starting with the bottom step and working up.
The form for the exterior of the wall would be far simpler.
Set riser boards to the marks and fasten them to the walls or side boards.
To form a concrete wall first set up plywood boards on either side of your concrete footer and attach them together with wire.
You can do this by laying each stringer form so the even bottom side of each stringer is flat on the ground and the zigzag side of the stringer is oriented with its points facing up.
Tap the sides of the forms and risers with a hammer and jab a 2x4 up and down in the mix to drive air bubbles out.
A monolithic pour would give you the best outcome.
Building formwork when forming between walls or other flat surfaces such as plywood where you will be able to mark riser locations positioning risers is easy.
A cap on the wall would be needed and you would want a thin mix on the wall but a thicker mix on the steps that you would pour first.