Simple. Take apart all the fittings on the keg. Sanitize them, as well as the keg and any transferring equipment you'll use (siphon, hose, etc.) just like when bottling.
2. Replace all fittings on keg, except for lid. Drain keg of sanitizer, rinse if needed per sanitizer's instructions.
3. Fill the keg.
4. Install lid in keg.
5. Attach the GAS IN line only to the IN post of keg. Set CO2 regulator to 30 psi. You'll hear the gas going into the keg for a few seconds. If it continues to audibly enter the keg for what your gut says is too long, you probably didn't get a good seat on the keg lid. Remove the gas line from the keg, release pressure via the release valve in the lid, and remove/ reinstall lid. Repeat procedure.
6. If you're in a hurry, leave the gas connected at 30 psi. for a few days, keg in the fridge. Turn down to serving (~12psi) after 3 or so days. If you're not in a hurry, after 'setting' the lid turn pressure down to 12, fridge it, and come back in a week or 2.
I never bother with purging oxygen from my kegs and have never had a problem. To me it's a waste of gas. I also never bother with naturally carb'ing kegged beer- mostly because I want to drink it sooner and am not interested in the added trub of natural carbonation.