Hi guys,
I need help with Plumbing guidance with above, as frankly the lack of instruction regarding the Plumbing on the product is shocking (see full packaged instructions below).

I'm trying to figure out how to attach the packaged drainage assembly to a P trap

I'm guessing the hexagonal fitting on the thread goes below the shower base along with the rubber seal to form a tight bond with the base and the rest of the outlet system. First, Shouldn't there be a seal for the top section as well? Logic would dictate so? Secondly, how can you get this to occur as I can't find any couplings/unions that will allow this to tighten as I screw this assembly from above once I lay the base down.
Thanks in advance!