Here is what Ford recomends.
Other methods may work. That is your choice... System Charging Set-Up
Ford Motor Company recommends using a charging station to perform evacuation and charging of the refrigerant system. Follow the instructions provided with the charging station.
If a charging station is not available, system charging may be accomplished using a separate vacuum pump, charging cylinder and manifold gauge set. The use of small cans of R-134a is NOT recommended.
If charging cylinder method is used, the center port of the manifold gauge set should have two refrigerant hoses with integral shut-off valves built into the manifold gauge set. If the gauge set does not have shut-off valves and two hoses at the center port, a tee fitting and two hoses should be installed at the center port. In addition, the hoses attached to the center port should have shut-off valves at the other ends of the two center hoses to prevent air from entering the hoses when not connected to the vacuum pump and charging cylinder.Service Access Gauge Port Valves
The low-pressure service access gauge port valve is located on the suction accumulator/drier between the A/C evaporator core and the A/C compressor.
The high-pressure service access gauge port valve is located on the A/C manifold and tube (19D734) on the 3.8L engine and on the A/C compressor to condenser discharge line (19972) on the 4.6L engine.
Service Access Gauge Port Valve Locations -- 3.8L Engine (4.6L Engine Similar) Charging from Small Containers
The refrigerant charge level of A/C systems is critical to optimum performance. Either an under-charge or an over-charge will adversely affect performance. Using small cans to charge these systems is not recommended because the charge level cannot be accurately controlled. A charging cylinder or a charging station is the only recommended method. Charging With a Charging Cylinder
1. CAUTION: Liquid-charge into the vehicle suction accumulator/drier (19C836) only. To prevent A/C compressor slugging, do not liquid-charge into a refrigerant hose while the engine is operating. Slugging can damage the A/C compressor (19703).
If the vehicle suction (low) side service part is located on the suction accumulator/drier, connect the gauge set center hose to the liquid port of the charging cylinder. If the suction (low) side service part is NOT on the suction accumulator/drier (located on the suction hose), connect the center hose to the GAS port of the charging cylinder.
2. When evacuating the system with the vacuum pump, the gauge set second center hose should be connected to the charging cylinder and opened to the gauge set so that the hose will be evacuated with the system.
3. When evacuation of the system is complete, close the center hose valve to the vacuum pump and turn off the pump.
4. Open the charging cylinder valve and the gauge set low-side valve to allow refrigerant to enter the system.
When no more refrigerant is being drawn into the system, start the engine and select an A/C function on the control assembly. Select the highest blower speed and allow the remaining refrigerant to be drawn into the system. Continue to add refrigerant until the specified weight of R-134a has been added, then close the charging cylinder valve and allow the system to pull any remaining refrigerant from the hose. When the suction pressure drops to approximately 207 kPa (30 psi), close the gauge set center hose valve.