The usual recommendation is to add the amount of oil that you were able to drain from the old components. This amount might be close to what you need but you might want to add more to account for the oil that does not drain from the old parts, plus the amount that was lost if you had any leaks. Even if you had a leak you should add less than the total oil capacity of the system because some oil should remain in the compressor. As you can imagine this is very imprecise and involves some guessing.
Add the oil first, then the refrigerant.