I'm having a partial inground/above ground pool installed with vinyl lining. I need to install a p-trap in the clean-out thingee in case I ever have to drain it with a hose. How can I install the p-trap?

P traps are needed when you are connected to a sewer drain. Are you planning to drain your pool into your sanitary sewer rather than out into the yard. Most pools are never drained unless you are replacing the liner which should be 5-10 years down the road