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.
Assemble your concrete form.
Use screws and a power drill to attach each riser form board to your stringers.
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.
You can do it like a wall build a wall set your risers and false form the inside of it.
Shovel the concrete inside the forms starting with the bottom step and working up.
The inner side wall form would be your tricky cut but could be braced one to the other.
Keep in mind though that if the finished exposed product is going to be concrete you ll need to strip all of this out finish it all before the concrete sets completely.
To form a concrete wall first set up plywood boards on either side of your concrete footer and attach them together with wire.
Set riser boards to the marks and fasten them to the walls or side boards.
You would cut a form for the inside that would be similar to a stair stringer but set upside down.
If steps are longer than 4 feet install stringers to support risers and keep stair lines straight.
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.
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.
A monolithic pour would give you the best outcome.
The form for the exterior of the wall would be far simpler.
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.