(a) Use a teflon tape sealant or (b) a thread paste sealant. The right sealant for threaded joints is non-hardening, compatible with plastic and doesn’t add slipperiness to encourage over-torquing.
Screw pipe or fitting into Ball Valve, hand tight.
Tighten pipe or fitting another one to two turns until seal achieved. Do not over tighten.
Turn handle clockwise slightly.
Put fingers under handle and pull whilst pushing with thumbs on body of valve.