I am remodeling by bathroom. I am adding a new tub and the directions say to put a concrete slab under the tub and I would like to know how to make it. Currently the floor is down to the slab what do I do?

If this tub is plastic or steel I think they are suggesting that you bed the tub in. What material is the tub? Let me explain about bedding a tub.
A fiberglass or plastic shower stall or bathtub should be bedded, which means support is required under the base so the floor does not flex or give when someone is standing on it. If the unit is installed without proper support then in time the drain seal will rupture and cause a leak. Putting cement or mortar under the floor when it is installed gives it a solid platform , which prevents flex in the floor. Simply puddle or make a ball of thin set, cement or mortar and place them around under the tub and bear the drain. Without this support, the flex will cause movement at the pipe connection resulting in cracks in the base and leaks at the drain. Good luck, Tom

What kind of tub is it ? ( tin, cast iron, plastic... )

In the interim: you mix light weight concrete, put elongated blob in the middle where tub will be standing and slowly lower tub on top of it. Weight of the tub will squeeze concrete out and leave under neath exactly what it need, filling space between bottom of the tub and floor. Try not to move the tub or work on it until concrete sets. Then, continue with your remodel... Hope that helps... Milo

I agree, I also normally fill the tub half way and allow it to sit for a few days.