Put on your safety goggles and gloves.
Connect the red hose from your AC gauge to the service port on the smaller of the two pipes coming out of your unit. The service port is located on the back of the unit.
Connect the blue hose from your AC gauge to the service port on the larger of the two pipes coming out of the unit. This service port is located next to the high side service port.
Connect the yellow hose from your AC gauge to your Freon tank. Turn the handle on the top of the Freon tank to open it.
Open the blue handle on your AC to release the Freon into the system. Wait for the needle on your AC gauge to reach the green zone. This means that your unit is charged properly.
Disconnect the gauges and the replace the service port caps.