A properly soldered (or "sweated") joint on copper pipe is as strong as the pipe itself. A poorly soldered joint will leak. Work systematically because each step depends on the previous one: The pipe must be cut straight and all burrs removed. The inside of the fitting and the outside of the pipe must be clean. Flux must be applied for the solder to adhere. The solder must be fully drawn into the joint.
Pipe ends and fittings must be perfectly round. If either is dented or even slightly flattened, it is all but impossible to restore the original roundness. Cut the pipe again or buy a new fitting.
Cutting with a tubing cutter ensures roundness. If space is tight and you must cut with a hacksaw, do it slowly and gently. If you must bend a pipe to move it away from a wall, work carefully.
About 15 minutes to cut a pipe and join a fitting
Felt-tip marker, tubing cutter or hacksaw, multi-use wire brush, flux brush, propane torch (preferably with a trigger igniter), fiber shield
Cutting pipe, soldering
Protect any flammable surfaces with a fiber shield or cookie sheet.
Copper pipe and fittings, flux, solder (95 percent tin for drinking water supply), damp rag
Use a large tubing cutter or a small one if space is tight. Align the cutting wheel with the cut mark. Twist the knob until the wheel starts to bite into the pipe. Rotate the cutter once, tighten a half turn or so, and repeat until the pipe is cut. Assemble all the parts of a joint in a dry run.