A compression tester consists of a pressure gauge, a non-return valve, a pressure release valve, and an adaptor to allow it to be connected to a spark plug hole. You can get one at just about any automotive shop. Some come with a rubber cone that you press against the spark plug hole, and others have a flexible hose with a spark plug thread on the end of it, so you screw the fitting into the spark plug hole. That kind seems more favoured by professionals than the rubber cone type, because it is easier to use single-handed: it can be a bit of a challenge to spin the engine whilst holding the gauge against the spark plug hole with one hand. There should be instructions with the gauge - it is simple to operate but there are hints as to what various odd readings mean.
I currently use a Ryco one and it seems satisfactory. For decades I used a plastic American one that cost about a tenth as much and gave the same results. I think I finally threw it in the skip after I inherited the new Ryco one.
