Open the valve before heating it to protect the rubber seal from being damaged. Flux all four ends and fit the shut-off onto the pipe. Position the valve straight and heat and solder each joint. Wipe away any excess solder with a rag. Turn off the main water supply and cut off the end cap of the copper water line, leaving enough pipe exposed so the shut-off will fit properly.
Test fit the valve over the line and position the ring to meet the threads.
Screw on the compression nut by hand. Then tighten it down firmly with an adjustable wrench while holding the valve in place with another wrench.