Use the same soldering techniques to install the new assembly to the water line.
Depending on the installation, it may be necessary to use inexpensive copper fittings to connect new sections of pipe to the existing water line. These fittings are widely available in different configurations and diameters.
Replace the petcock, and open the main water line to check for leaks.
Before temperatures dip below freezing, shut off the valve, and open any spigots that are "downstream." Then remove the petcock screw to drain the pipe.
If any sections of pipe "upstream" of the new valve are in an unheated area, cover them with foam pipe insulation.