I do something similar to what BrewBama does but on a smaller scale. I use a 20 mL syringe to inject the fining into the gas-in fitting:
1. Connect a short piece of tubing to the syringe.
2. Suck up the fining solution.
3. Attach a gas ball-lock connector to the open end of the tubing.
4. Connect a carb cap to the connector.
5. Push the plunger just enough to push out all the air, which you'll know when you see liquid in the stem of the carb cap.
6. Remove the carb cap, connect the gas ball-lock connector to the gas-in fitting of the keg and push the rest in.
More often than not, though, I just open the lid and dump in the fining. I keep it on 1-2 psi of positive pressure when the lid is open in an effort to minimize O2 ingress. Works well.