Pressure Pot's are not the most common things around, ours here at the shop is from GMC, but there are many others out there designed for scuba as well as watch testing. Heck Diver58 in the post before yours built one out of a pressure cooker which is just awesome in my book. The process I use is as follows:
1. First make sure you do a very careful and clean battery change, the best way to have a good pressure test is to not have the computer leak. Clean everything up well and take your time.
2. Fill the pressure pot with about 4" of fresh water and set the watch to be tested in the bottom. I attach it to a dive weight to make sure that it stays in place and stays submerged. Suunto recommends on the D6 and D9 that the computers are not tested in air as it may damage the sensors. You may also place a reference sample in the pot too (another D9) to check the uniformity of the readings.
3. Take it on a "test dive" to the depth of your choosing, I use 130' as my standard. Making sure that the depth readings are accurate both on the way down as well as on the way up. You also want to look very closely for any signs of a leak, visible water, fogging, dimming of the screen etc.
4. Once the dive is over dry the watch completely off with a soft cloth and set it next to a mild heat source, a desk lamp or small flashlight (even just wear it outside on a warm day for a while). This will show if any water has entered by causing the screen to fog if any moisture made it in. Remember we are not cooking D9's here just let it warm up a bit.
If you want to be even more cautious you can take the computer on a really shallow dive, say 30' then warm it up and look for signs of leaking. Then if everything is ok, take it on a deeper dive. The thought is if it leaks it will at least have leaked less helping to minimize any possible damage and letting you get into the watch more quickly to clean things up.