How do you go about constructing stepped foundations?

Stepped foundations require no more than creating forms which step down in the dimensions you desire. Two special notes though. Bend rebar (heat slowly and bend slowly) to conform to the step down. And, pour the lowest portion of the foundation first, then screw down a plywood lid over the lower form, before pouring the higher step up portion.

This prevents the weight of the concrete in the higher form from forcing a bulging up of the concrete in the lower form, due to the weight pressure. Be sure the lower forms are firmly anchored to stakes which are anchored in the ground, to prevent the pressure from the concrete in the upper forms from lifting the lower forms.

As the downward pressure in the upper form is transferred to the concrete in the lower forms, it exerts a lifting effect on the concrete in the lower form pushing up on the lid of the lower form, as the concrete in the upper and lower forms try to equalize their levels.