This what I do:
-Cold crash my carboy for 48 hours
-Disasemble and clean my keg with the cleaner of choice..... PBW, Oxyclean, etc
-Sanitize all the parts and reassemble
-Purge my keg with CO2 the best I can
-Transfer my beer into the keg
NOTE: If you are planning on dry hopping, additional flavorings, other alcohol (Irish whisky, Burbon, etc), coffee....you can do this now.
-Careful not to allow oxygenation to the beer
-Once full, close it up and apply 25 lbs of pressure.
-Purge and oxygen out of the head space. 5 or so purges from the bleeder valve on top of the keg should do it. If the pressure drops slightly over the next few days, just turn it up and keep it at 25psi.
- Give the keg a slight roll around just to blend in some CO2 for a few minutes wether that be on the ground or in your lap.
-Let it sit for 3 days and back the pressure off to serving pressure. For my IPA's and Pale Ales I keep it between 8 and 10psi.
After this point, the longer you let the beer sit, the better CO2 will absorb into your beer. At this stage, your beer will be carbonated, but it will lose carbonation quickly while in the glass. Give it a week or so and your golden.
Obviously different styles may need longer times to condition than others, but you'll have to use your judgement on that.