I clamped a PVC threaded cap in the intake boot with a pressure gauge and a valve stem screwed in the end of the cap.
The exhaust side I plugged with a rubber expansion plug right in the exhaust port (y-pipe removed)
You will also have to plug your VES hose
Here is a quickly scabbed pic of a random setup, not mine and slightly different.
Everyone has different pressure recommendations, but apply pressure slowly and carefully! I test my engines @6psi for 10 minutes. Any engines that have leaked on me did so VERY quickly... all pressure lost inside of a minute. If they are sealed well, you should see no loss through ten minutes.
If you have a leak, you can use regulated air pressure, and a spray bottle of soapy water to find the problem. If a center crank seal is gone, the leak will bleed into the center cavity, and out its supply hose.
Another thing to keep in mind is that my method does not test the y-pipe gaskets or the y-pipe to pipe seal.... these are also critical sealing points that can essentially be intake leaks as well.
I leakdown test EVERY time I assemble a 2stroke engine.