I did all the plumbing in this house. All the shutoff valves are ball valves with teflon seals. (Look into the valve. Teflon seals are white.) I soldered all the connection and had zero leaks or failures.
Clean both the valve and the copper pipe with steel wool, apply flux, fit together and solder with a propane torch. Apply heat to the joint area until the solder melts on contact with the joint and flows into it.
When soldering, cleanliness is not next to Godliness. It's way out in front.
Ted