I'm in the KISS plan club, this process will carb it, but not to perfection, and the BJCP gods will frown with very serious faces as they puff on their pipes whilst carefully ensuring the head on their Porter is appropriate to the style guidelines all the time tut, tutting....
1. Chill
2. Crank it up to 40 psi
3. Shake Keg for 5 mins (you'll hear the regulator releasing the gas into the keg as the beer absorbs the CO2)
4. Reduced the reg pressure
5. Release the head pressure using the pressure relief valve (have towel ready to stop spraying of foam out of valve)
6. Put 5ish lbs pressure on reg
7. Serve (it will be foamy at first, it will get better)
Many folks let it set for a few hours after doing this, but I just deal with imperfection of it all. You won't be able to get precise volumes of CO2 doing this, it will however be drinkable.
Having said this, my current experiment of fermenting under pressure is pre-carbing my product naturally and the first keg is getting very interesting, and is being crash cooled right now.
JQ