You are basically pressurizing a closed system.

1. Turn your gas off.

2. Close the outlet valve on your brewpot/fv.

3. Pull the pressure relief valve on your keg lid.

4. Remove the lid from your keg.

5. Turn the gas on.

6. Purge the keg with CO2 from the gas in line with the lid off.

7. Close the valve on the manifold leading to the gas in on the keg.

8. Open your fv drain valve & fill the leg with the lid off.

9. Secure the keg lid.

10. Open the CO2 valve on the manifold and pressurize your keg at appropriate psi for your beer style.

Purging the open keg with CO2 will create a blanket of CO2 on the surface of your beer as it fills the keg. So long as you don’t move too much air around, and keep a nice, slow, even fill rate, you will not oxidize your beer.