We use composite plastic door shims if the floor is not level.
Shim near the back, slid the shim in, score with a knife, and snap off.
It takes two seconds with door shims.
Score and Snap!
The caulk around the front with clear PolySeamSeal.
Don't use Silicone, grout, plaster or putty.
Putty is just some clay with oil in it.
It's the worst thing you could use.
Clear PolySeamSeal, it's in the painting department.
The shims are in the door department.
Don't buy shims in the plumbing department.
Plumbers don't use them, they take about fifteen minutes to slice off.