There are two basic types of soldered capillary joint, an End Feed Joint and an Integral Ring Fitting. In the end feed joint, solder is fed into the end of the end feed joint and capillary action feeds it into the fitting. With an integral ring, the solder is already contained in a reservoir (ring) on the fitting.
Measuring and cutting
Carefully measure the length of copper pipe, making sure you take into account the extra needed to fit inside the joint. Hold the pipe firmly and cut the end square with a fine-toothed hacksaw or a pipe cutter.
Clean the inside and outside of the pipe where it has been cut with a round file to remove any burrs. Some pipe cutters have a blade for removing burrs, but a fine file will do the job just as well. Wipe the end of the pipe with a clean cloth. Copper pipe needs to be completely clean for the solder to take properly, so you need to brush the end with Flux (a chemical cleaner) to remove any grease. Coat both the ends of the pipe and fittings with the flux, and then push the ends of the pipe into the fitting until they stop.
If using an Integral Ring joint, move the flame of your blowtorch evenly around the joint until a ring of solder appears at each end of the fitting. With an End Feed joint, heat the flux until it bubbles and then touch the end of the solder wire to the end of the fitting. Once a complete ring of solder can be seen around the edge of the fitting, the joint is sealed.