You find the size closest to the size of the tubing and then the little ****** on the tool pushes into the coupling. You can only do it on one side of the coupling, so you will need to make sure you are on the correct side. You might feel a little snap when it goes in. While it is pushed in, the lines shoud be able to be pulled apart. Sometimes they are a bit sticky, and it helps if you can rotate it even a little bit to free it up. I have had about 10% of them be a real pain, the other 90% are usually very easy.
WHen you put them back together, you do not need to use the tool. Just lubricate the o-ring with the CORRECT oil for your A/C system, and the tubes will slide together. You will definitely feel a snap when they lock in place.
Good luck
- Mike