Or, to keep it simple, just "pour" some CO2 in the keg when filled and just before you put the lid on. This can be done two ways: With a hose on the CO2 bottle with an in-line shut off. The other one is a trick I learned from one handy brewer.... he carries one of these:
Minus the ball connect and uses it as a six shooter in his back pocket..... just a squirt in the keg and hoppa, the lid goes on.
Or, just thought of this.... (I have one of those) you could just leave the ball connector on, hook it up and shoot in some CO2 with the blow off vent open, that would purge it....
There's a lot of ways to do this......none of them too complicated.