To solder copper and get a water tight joint, consider first these factors that will prevent your solder job from working correctly: dirt on the paste or on the copper itself, too much heat that can burn the flux off, and water.
First, clean the pipes completely of dirt, oils, and oxidation that may prevent the solder from sticking. You can use sandpaper and grit cloth, and, if the copper pipe is new, steel wool. Next, cut through the tube preferably with a tube cutter, but a hacksaw works fine, too.
Coat the pipe and fitting all over with flux, then heat the joint. Next, assemble all the pipes to check accuracy of cutting and fitting, solder the joints one after another. Heat the joint to melt the solder, but not too hot or it will overheat the flux. Brush more flux on areas you see that need more solder.
Move on to the Next Page video: How to solder copper pipe. Tips and tricks!