Bob, the easiest way is with an adapter that screws into the spark plug threads and connects to your air hose. I have one of the cheapo leakdown testers from Harbor Freight. One of the adapters in the kit fits the sparkplug holes for our engines and snaps into the quick release on my air hose.

https://www.harborfreight.com/cylinder-leak-down-tester-62595.html

I have seen one made from an old spark plug that had the ceramic knocked out of it, got hollowed out, and then was somehow tapped for a quick release air fitting.

Matt