Compression joints are made with a fitting that clamps a compression ring onto a pipe and at the same time wedges it into the fitting. There are a few basic rules to follow when attempting to form a sound compression joint.
First, push the nut onto the pipe, and then push the brass compression ring up against the nut. The end of the pipe then needs to be inserted fully into the fitting, ensuring as you do this that it reaches the stop.
Next, push the compressing ring along the pipe to the mouth of the fitting before winding the nut onto the thread in a clockwise direction. This will have the effect of pulling the compression ring right into the fitting, but the compression nut should not be too strongly tightened; otherwise, the compression ring inside will become distorted and possibly cause a leak.
The joint should be tightened only enough to hold the connection firmly. It can always be tightened later when the water has been turned back on, but if it is too tight at the outset, it will not be possible to tighten it further should a leak develop.
One of the advantages of compression joints, apart from their reliability, is that you do not need to use any jointing materials with them. This makes them less messy to form than some other pipe joints. Plumbers sometimes wrap polytetrafluoroethylene tape around the joint, however, or apply some jointing paste, especially if the compression ring is old and somewhat worn. The jointing paste will make up for any blemishes, and the overall effect will be the same as forming a brand-new joint. The paste is not applied directly to the threads of the fitting because these do not actually form the seal but only pull the joint together.