Take the largest bucket containing foubdation mortar intended for your foundation.
Place 4 to 7 bricks and place reinforcement bars (wire mesh) on top of them to strengthen the foundation.
Pour 150 ml of water into a bucket with foundation mortar and stir until there is no dry mixture left. Use a metal spoon or fork (don't worry, it will wash off easily).
Pour the mixture onto the base, pressing gently and smoothing with a larger trowel. Add the mixture to the hardened stair formwork, pressing gently so that the cement fills all the gaps.
Use a trowel to level the foundation as much as possible, form steps so that the cement is level with the top of the wooden boards.