There are two ways to do this: To do it correctly, need to draw the system down to evacuate it and hold a vacume for awhile, if it wont hold a vacume, the system is not tight and has a leak. You will need the guages and a vacume pump for this.
Assuming theres no leaks, (if you cant hold a vacume, partially charge system with a dye) If the system is tight, add the proper amount of refrigerant without releasing the vacume. Too much freon will also cause a warm air condition. The compressor will cycle on and off intermittently when operating normally.
The car has Low and Hi psi sides. The larger diameter lines on the car is Low, the smaller lines are high side psi. (The pressure drop makes the freon cold at the Thermal expansion valve at the firewall, you can feel the cold pipe when its working properly). Always add freon to the Low side.
Note: The guages have 3 hoses, Red for Hi, Blue for Low and the third hose (can be any color) is for accessory, like a vacuum pump or adding freon. Each guage has a valve, opening the valve will allow the 3rd hose direct feed (or draw) from the accessory hose. All freon is as added to the Low side guage. This guage is a compound guage and reads inches of mercury (hg) past 0 psi, (negative psi or vacume).
Its a doable project, if you have the right tools.
Now that you know the theory:
If you dont have the tools, check for leaks with a dye and remember, the evaporator (like a heater core) can be leaking inside the dash). If you add refrigerant w/o a sealed vacume tight system, it may work, but never as cold as it is capable of blowing.
Good luck.