Leak down test is basically putting a threaded extension (bit like your compression tester) into the spark plug hole and then pressuring the cylinder with compressed air from your handy household compressor

(Not sure if you have access to an airline or not)

The leakdown test will basically show how much of a measured pressure is lost.

You can listen to where the air is escaping by listening either at the inlet or at the exhaust tailpipe - or through the oil filler cap if you have serious issues