As I mentioned, the old compressor should be removed. New sealing rings should be installed on the refrigerant line set which bolts to the back of the compressor. The new compressor is loaded with about 2 oz. of PAG oil then the compressor clutch is rotated by hand to distribute the oil. The new compressor is bolted into place and the belt is installed. All electrical connectors should be connected to the compressor and clutch.
The low pressure cutout switch is removed from the accumulator, then the low pressure line is removed from the accumulator and the accumulator is removed from the evaporator line so it can be removed from the vehicle. The high pressure line at the evaporator is removed and old orifice tube pulled out of the evaporator inlet. The new orifice/screen is installed, then the high pressure line is reconnected to the evaporator. The low pressure side of the evaporator is connected to the new (or cleaned) accumulator. The remainder of the oil charge can be added to the accumulator then the low pressure line to the compressor is reconnected to the accumulator. Alternately, the oil can be added with the refrigerant, but measuring it into the accumulator is a lot more accurate, and PAG oil is hygroscopic (will absorb atmospheric moisture) so it is important to install the oil charge before evacuating the system.
Once the system is sealed the manifold gauge set hoses are connected to the high- and low-pressure fittings and another hose is connected from the gauge manifold to the vacuum pump. The pump is started and the manifold valves are opened to subject the entire system to vacuum until at least 27" Hg of vacuum is achieved and maintained for ten minutes at a minimum. This will help purge the system of any moisture and air, and establish a test point for vacuum testing. The pump can be shut off and the gauges monitored for another ten minutes to make sure the vacuum holds without dropping (indicating leakage into the system and eventual leakage out of the system once refrigerant is installed).
If the system holds vacuum without leakage the vacuum pump can be disconnected and the refrigerant supply connected to the gauge manifold. The weight of the refrigerant cylinder is recorded and the engine is started with the A/C controls set to full cold. Refrigerant is added until the correct mass of refrigerant is applied and/or the system pressures are adequate for the given temperature conditions and system superheat.
Pretty easy, no?