When you've got a bathtub printed you'll want to make sure it's sealed all around and won't leak. I'd recommend acetone smoothing if you're going with FDM printed parts. For the powder prints I applied varnish and then floor wax to the mold to make sure no silicone soaked in. You'll want to give the FDM a light coat of wax or mold release as well.

Once your mold release is dry, go ahead and mix up some silicone and pour in a thin even stream into a corner of the mold until it's full to the top. I wrote a tutorial on silicone casting a while back that you might like to take a look at.

When your silicone is fully cured (this usually takes about 8 hours) demold it. You may have to get in there with an xacto to free up the little pyramids on the ends of those straight ribs, but those cuts will also help get the waxes out in the next step, so don't worry.