Compression fittings work on both rigid and soft tubing, most often used for under-slab water piping and in-house gas piping. The goal is to expand-flare out-the end of a soft copper pipe to match the male end of a typical flared fitting. The most common type of flaring tool has vise to hold the pipe and a separate flaring head that clamps to the vise and turns into the end of the tube. Once the ends is flared the tubing nut is threaded onto the fitting.

The tools and materials you will need are: Tubing and fittings, tubing cutter, pipe joint compound, tubing bender, groove-joint pliers, clamp flaring tool, adjustable  wrenches.

Step 1: Bends in soft copper must be made carefully, or the tubing will kink. The best approach is to use a spring-type tubing bender. This tool just slides over the tubing, and you make the bend with  your hands. The spring distributes the force over the entire length of the bender, so no kinks form.

Step 2: The vise base on a flaring tool has two parts. To use it, just slide the end of the tubing into its propersized hole and tighten both wing nuts securely. Be sure to slide  the fitting nut onto the pipe before attaching the vise, and make sure the tubing extends 1/8 in. above the top of the vise.

Step 3: Slide the flaring head onto the vise, and make sure that the tapered end of the stem fits over the end of the tubing. Once this stem is seated properly, turn the stem in a clockwise direction to start flaring the end of the tubing. Work slowly to ensure an even flare.

Step 4: Continue turning the flaring stem into the tubing. Generally, the flare is complete when a lip that is about 1/16 in. wide all around  is formed, You don't have to measure this. Once the copper fills the tapered opening in the vise, the flare is done. This job usually takes a little trial-and-error to get right. 

Step 5: Soft copper tubing is often used to connect gas appliances to standard steel gas pipes. The flared fitting has pipe threads on the both ends; install the standard (non-flared) end in the steel fitting. Apply a light coat of pipe joint compound to the threads before tightening the fitting in place.

Step 6: The free end of the flared fitting should be coated with pipe joint compound. Make sure to cover all the threads, but don't get the compound inside the fitting. Compound that spills into the fitting can move through the line when the gas is turned on and clog the orifices where the gas is burned.

Step 7: Slide the nut to the end of the copper tubing, and press it against the fitting. Carefully thread the nut in place with your fingers to avoid crossing the threads. Don't switch to a wrench until the nut is at least  halfway onto the fitting. Finish up by firmly tightening the nut with an adjustable wrench.