I have a toilet in the basement with proper vent. I want to add a bathtub near the toilet. After I jackhammer up the concrete and install the tub trap and drainage into the toilet drain, how do I seal the floor under the tub? The outside part is open and it should be easy to pour concrete mix and seal the slab before tiling, but the inside part, directly underneath the tub would be difficult. If it is not sealed properly, water would seep into the basement. Did I miss anything obvious? If I did not explain this problem clearly, would you give me the correct sequence of this installation job?

Thanks in advance.