I'm not a pro but it just so happens I installed my own cast iron tub just yesterday!! Here's what I learned -- first, in this instance, the instructions (it's a Kohler Villager) said not to hang the tub off its rim along the walls. It's supposed to sit on the four feet underneath. Once we got the tub situated in the alcove, we realized it wasn't level... so we used shims to level it up, checking along the back wall and along the two short walls. Get it as level as you can, but something tells me these cast iron tubs have oddities that make absolute perfect level kind of impossible. But we got really close! It was also important to measure from the front apron to the wall across to make sure the tub was sitting parallel, and square to the rest of the room. My walls are still open so I had an advantage with access for shoving mud underneath... So, if you have access it's good to shove mud underneath for added support. I used something called Cement All, Rapid Set. It's a form of cement grout that doesn't shrink as it dries. I mixed it stiff and made a mound of this stuff underneath, like a cement spine. Make sure you reach under and smush it so the cement is pressed up against the belly of the tub. It sets up in only 20 minutes, but that was more than enough time. That's it! It wasn't so bad.