Ball valves are reliable and long-lasting and great for such uses as a toilet shutoff. But you need to take some extra care when sweating, or soldering, them. Nylon or teflon linings can be damaged by the heat, at temperatures slightly beyond those needed to solder. That means the soldering has to be quick, clean and professional.
To prevent a night without water (and heat from the family), it might be an idea to start by doing the work when the store’s open, or pick up an extra valve (or two); they’re inexpensive and you can always return them.
A torch using MAPP gas is a good idea; it will heat the fitting faster, so there’s less risk of overheating the valve for a long period of time. (Like propane, it comes in a cylinder, but you need a special MAPP torch to get maximum performance from it.)
Make sure all water is out of the pipe before applying the fitting by heating the pipe. If you heat it to disperse the water after putting the fitting on, you’ll heat the valve unnecessarily.
Clean, flux and fit the valve as you would any fitting.
Some recommend shutting the valve, so as not to draw heat through it, but the backpressure can fight the solder from being sucked in. You can good results with the valve open.
You may want to place a small bit of solder on the pipe as a bellwether. It will help you know when the joint is hot enough.
Heat the fitting near the pipe until you begin to see a green flame on the copper pipe, or if you’ve used a piece of solder, when the solder melts. Hold the flame for another couple of seconds, then remove it and apply the solder. If you’ve done it right, the solder should disappear – sucking right into the joint.