How do you install a union on a pipe?

You carefully cut the pipe, then if you can push it apart enough to fit one nut and one'olive on each end of the pipe. Now seat both ends in the union. At this point you hold the centre of the union with a wrench and tighten each end nut while pushing the pipe in towards the union. When both ends feel tight, hold each end nut with a wrench and give one final 1/4 turn. That was with a standard brass union, now you can also fit a 'sharkbite' with no wrench, just firmly pushing each end in. With 'sharkbite', however you MUST make sure the pipe ends are smooth and have no sharp edges.