How do I install a P-Trap into an existing bathtub? I am somewhat savy with plumbing, but it is in a tight space and I am looking for some advice.

The only way you are going to install the trap is to open the ceiling from below or open the adjacent wall at the drain end of the tub.

How do I install a P-Trap into an existing bathtub? I am somewhat savy with plumbing, but it is in a tight space and I am looking for some advice.
Surely there must be some sort of trap already on the tub.

A drum-trap perhaps?

If it's already trapped, and is working -- Then don't @&%$ with it.