CLT, or Cylinder Leak Down, Test is preformed with the same basic stuff you use for the compression test. You need to pull the valve cover and crank the engine by hand (with a wrench) until you see both valves are fully closed on the cylinder you will be testing (piston needs to be top dead center, intake valve will open then close, and then go about 1/4 rotation on crank). You then have someone hold the crank in position (it will move so use a big breaker bar on the snout bolt, with a big friend) and you push compressed air into the engine at around 100 psi using compression kit adapter with an air fitting. Then you get listen for where its leaking from. Pull out the oil dipstick, if air is coming out you have bad rings or scored cylinders. Open the throttle plate, if air is leaking out the intake you have a bad intake valve, exhaust - bad exhaust valve. All the plugs should be out while you do this, if air is leaking out of the degas bottle for the cooling system or the adjacent cylinder then you have a bad head gasket. You should then be able to tell if you need head work (valves), base engine (pistons, rings, cylinders), or a head gasket. If you need a head gasket you'll need a straight edge (machined) and some feeler gauges to make sure the block and/or head isn't warped. The test will tell you with a good deal of certainty what, if you want to try a rebuild, or if you want to look for a secondhand running engine and swap 'em out. (Just keep the new ignition stuff and transfer it over as a second hander would most likely need it also). But yes, don't crack it open until you know what and why you would be doing it.