How To Perform a Compression Test

Don't just remove the gauge, there's a lot of pressure in the line and you want to release it first. Thankfully they thought of this, and there's a little button on the side. Depress the button and you'll hear the pressure hiss out. Now it's safe to remove the gauge, unscrew the testing tube, and take out the adapter.

Replace the spark plug and repeat the whole process on the next cylinder until you have readings for all of them. Check your repair manual to see if the readings you got are healthy.