What is the “correct” or “right” way to thread a PVC male threaded adapter to a Copper Female threaded adapter, and how or what do you use for sealant?
I would wrap the male adapter with Teflon tape 6 or 7 times and then apply a Teflon based pipe dope on top of the tape. Carefully thread the male adapter into the female fitting until it is hand tight. Now use a wrench or channel locks and give it a couple of turns. Wipe of the excess pipe dope and turn on the water to test for leaks.
Given we are talking about a PVC adapter to copper threaded adapter I always get a bit nervous about ensuring that there is a good tight seal and so I recommend you use a hefty does of Teflon pipe dope in conjunction with the Teflon tape your are wrapping around the male adapter mate. Teflon is a wonderful material that is not very expensive so be sure to use a lot of it and liberally apply the pipe Teflon dope. I want that connection to be tight and if you are aggressive in the application of the Teflon, when you go to screw on the fittings…that is make the requisite turns to mate the two adapters you want those turns to be providing you with a good degree of resistance. Allow for some time for everything to set and when you do your water leak test, get your flash out and check carefully. Come back in about 15 minutes while the joint is under full water pressure and check again. If all is well, then give yourself a pat on the back, you have successfully mated a PVC threaded adapter to your copper female threaded adapter.
Now here is some more advice on a related subject. After you are satisfied all is well with this connection, since you are already in the handy dandy plumber state of mind, go check some of your other joints where adapters were employed or where the water is controlled by a valve. Check to be sure that you do not have any little leaks. Often homes will have small little leaks and you can go on for years without every discovering them. But it is not a good thing to have happening. Moisture is usually not going to help with keeping you on the safe side of keeping your home dry free and that is important because moisture has a way of attracting undesirable things such as varmits…we are talking mice and rats as well as other unpleasant conditions such as wet rot and mildew. So I recommend you go on a little crusade to correct all (if any) little leaks you can find throughout the home and the only way to do so is to go actively looking for them. Check your attic for any water spots and in areas where the water lines are visible. Look under all of your sinks and cabinets to check to see that you do not have any dripping valves. Visually inspect around the back of your washer where the drain hose is connected to see if there is any evidence of a problem there. Check at the base of your toilet to see if the toilet is not leaking.