Begin by cleaning the area and areas around the spark plugs before you remove them ( compressed air should be used), if available, other wise a small brush , or even a bicycle pump will work. The whole idea is to to prevent dirt from getting into the cylinders as the compression check is being done
Remove all of the spark plugs from the engine. Be sure to keep or arrange them in order so that you can read the tips sequentially.
Block the throttle wide open.
Detach the coil wire from the center of the distributor cap and as well take care to ground it on the engine block ( in the case of pre electronic ignition vehicles). Next use a jumper wire with alligator clips on each end to ensure a good electrical ground. On EFI equipped vehicles, the fuel pump circuit should also be disabled by removing the fuse. It always pays to be conservative and thorough.
Install the compression gauge in the number one spark plug hole
Lastly crank the engine over at least five ( 5) compressions strokes and watch the gauge. The compression of the engine should build up quickly on a health engine. Low compression on the first stroke, of the engine ,followed by gradually increasing pressure on successive strokes , indicates worn piston rings. A low compression reading on the first stroke , which does not build up during successive strokes indicates a leaking or leaking valves or a blown head gasket. ( a cracked gasket might also be the cause). Deposits on the undersides of the valve heads can also cause low compression as well. Record carefully in writing the highest compression gauge reading obtained.