Use polystyrene flooring pieces to form the ceiling of the basement and then add rebar.
The flooring is actually done in conjunction with doing the walls because the flooring both helps to hold everything square and also keeps the rain out while building the internals of the walls. The specific steps involved are here .
Install Carnation EPS flooring
The EPS floor section pieces slot onto the wooden beams.
Leave a hole for the stairs, view hole from basement, and the elevator shaft.
When using the Carnation Flooring system for decking and internal landings that do not have a wall to support it at the edge then manually cut a beam form at 90 degrees to the pre-cut channels so that an additional beam is formed along the edge of the deck. It will need a wooden concrete form board along the outside to form the outer part of the concrete beam. Hand rail posts and landing balustrades will be set into this concrete edge beam.
When placing rebar and pouring concrete you will need temporary walkways. These are wooden planks held above the concrete height by wooden cubes.
Install Rebar in flooring
Install as per structural engineering drawings.
Includes the rebar 90 degree bends used to tie the floor to the wall. This rebar needs to tie into the rebar at the bottom of the concrete beam trenches, so that means the 90 degree rebar is needed mainly low down in the connection to the wall.