2004 Pontiac Bonneville

Don't.

Take it to a shop. It's dangerous to discharge and is bad for the environment (yes, even R-134A, it's just not AS bad).

But if you're gonna drain it yourself regardless, the valves are both on the passenger side. One is on the canister next to the power steering unit and the other is on the line leading from it. Wear thick gloves, goggles, long sleeves and wrap the valve well with a few rags or something (basically avoid any skin contact, wash immediately if contact occurs) and depress the pin in the valve carefully w/ a screwdriver (or something similar) so the pressure release is slow to avoid losing control and seriously injuring yourself/others. This will discharge or "drain" the system. Make sure you've let all the pressure out of the system through the valves that u possibly can because if you're disconnecting fittings there will still be some pressure in the system regardless so use the same safety measures if disconnecting fittings.

But like I said, "Don't."