1. Measure the tubing to be cut and mark with a felt-tipped marker. If a fitting needs to be inserted into existing tubing (e.g., a T-joint for a branch off), a section of tubing must be removed. Sometimes, marks are provided on the outside of the fitting that indicate the location of the shoulder inside the fitting which the tubing butts up against when inserted. This aids in working out how much of the tubing needs to be removed. This needs to be reasonably accurate if copper tubing is being used as it can be difficult to pull the two ends of the tubing apart to insert the fitting if a too-short section is removed, or difficult to pull the two sections of tubing together again if too much tubing is cut out. Temporarily remove any clips holding the tubing so that the sections can be moved apart easier.
2. Cut the tubing to be joined to the correct length. Ideally, use a pipe cutter which gives a square, smooth edge. Alternatively, use a junior hacksaw. Try to cut the tubing square and file the cut edges smooth
3. Slide the compression nut over the end of the tubing followed by the olive ring
4. If plastic pipe is being used, push an insert into the end of the pipe. This prevents the pipe from being crushed by the olive as the compression nut is tightened
5. Push the tubing as far as it will go into the fitting
6. Tighten the compression nut by hand
7. Mark the nut with a felt-tipped marker, so you know how much you've turned it while tightening with a tool
8. You need two tools: one to hold the fitting and the other to tighten the compression nut. You can use open-ended wrenches, vice grips, or water-pump pliers. Make sure the tools are properly sized and grip tightly so that they don't slip off while tightening. In general compression, nuts should be tightened one half to one whole turn after you've tightened by hand. If possible, you can tighten by half a turn, then turn on the water supply and wait for a few minutes see if the joint "weeps." If this is the case, try tightening further. It's important to not over tighten as this can deform the olive, preventing it from sealing properly and causing a leak. Knowing how much to tighten a nut is learned by experience, and as a rule of thumb, once the nut suddenly becomes difficult to turn, it is tight enough.