I read your other post stating that the reason for a pad is to prevent squeaking. This makes no sense, since the base should not move at all once installed. Only "pad" I am aware of under a shower or tub is a mortarbed. This is mortar poured under the base to prevent the bottom from flexing when someone enters it. Plastic sheeting is laid down for moisture protection and then pre-mixed mortar is piled under the base and then the base set into it and secured.
If you have never made a mistake, you haven't done much.