The first step in any walk-in bathtub installation is prepping your bathroom to receive the unit. The first step in this process would be to set the floor drain in to correct place using the manufacturers cut sheet. Once the drain is set you want to level and finish off the floor, walk in bathtubs are a free standing bathroom appliance this is why it is installed on a finished floor.
The next step would be to install your water connection. The best way to do this is to 
recess the connections into the wall behind where the walk in bathtub will be installed using the manufacturers recommend placement. Most walk in bathtubs utilize a ½” hot and cold feed line that connects using flexible braided metal lines similar to a washing machine.
At this point you will want to make sure that the correct power source is in place if your walk in bathtub requires it.
Once the walls are finished where the walk in bathtub will be installed it is time to adjust the feet so the unit is at the correct pitch to drain properly. Once the unit is level you will want to connect the water lines and then the drain using a Fernco connection. This will allow you to disconnect the drain and slide the unit out if it ever needs maintenance.
Once the bathtub is installed you want to run clean water through the unit using all the features including jets, shower wands and taps to make sure there are no leaks. This also cleans out any leftover debris from the manufacturing process. As a last step many installers will use clear silicone to seal the bathtub to the wall. This prevents water from leaking behind the bathtub while using the handheld shower wand.
