Step 1
Shut off the air conditioner so it isn't operational while you are checking the refrigerant.
Step 2
Attach the two hoses from the gauge manifold set to the pressure ports on the air-conditioner system. The gauges are designed for specific coolants to give you an accurate reading, so if your system does have FREON(tm) refridgerent, the gauges should be designed for that. The hoses are designed for low and high pressure, with the low-pressure hose usually blue while the high-pressure hose is red. The blue hose connects to the larger port on the AC used for suction, while the red hose attaches to the smaller port used for liquid.
Step 3
Run the system for 15 minutes. This allows it to reach a stable operation state, so you get a more accurate measure of the refrigerant level.
Step 4
Read the gauges and check with your Carrier manual to verify that the levels are in the appropriate range, which varies by the size of your system. The readings are in psi, which are pounds of pressure per square inch.