Valves generally work in one of two ways: either a solid internal part rotates to open or block the flow of water, or a stem with a washer or seal squeezes down to stop water flow.

Screw on the new ball valve, install the old nipple and the new leather washer (use Teflon tape or pipe dope on the threads). Tighten the coupling nut, turn on the water and check for leaks.