what brew said,
remove gas out connector,
tip keg about half way over so that the gas tube is covered on the inside by the wort
cover the post with a glass
press down on the poppet with a knife or pen or some such technical pointy thing.
collect some wort in the glass to measure
Refractometer works best, but you can get a few ounces out each time to measure with a hydrometer.
I usually crank the pressure when there is about 8-10 points left, but have found that it really only takes about 4 points to self carb.
I transfer to a serving keg, instead of serving off of the yeast cake.
To do that,
cold crash for a day or two
attach a picnic tap to the ferment keg and draw off a couple of cups, should be running clear by then
put co2 in serving keg and purge out the air.
set the pressure in the serving keg to a couple psi below the pressure in the ferment keg
attach a hose with black connectors on both ends to the out port of both kegs, the beer should start to flow, slowly as possible is good.
attach your co2 tank reg gas to the gas in on the ferment keg
attach your spunding valve to the gas connection on the ruserving keg and bleed off the excess pressure as the beer transfers.
Your natural carb won't be lost this way, and if transferred slowly, it will usually be very clear.
The speed of transfer can be increased by decreasing the head pressure in the serving keg, creating a larger differential, slowing it down the other way is more difficult, so if it gets going too fast and starts picking up trub, I usually disconnect, put a little more gas on the serving keg and resume, after you've done a few, it's like riding a bike.
JQ