Since you mentioned you have broken the integrity of the system I would highly recommend having it vacuumed down(for approximately 15 minutes) then testing that the system keeps the same vacuum for at least 5 minutes. Pulling a high vacuum forces any residual water vapor to boil off. It is also used to see of the system is sealed.
You always want to replace your old drier canister if you have had it apart for any length of time. Most desiccant driers are long gone at 15 years old. They can only absorm approximately 1/2 tablespoon or water when new.
Although you might be tempted to use R12 the problem is that if everything is not perfect(seals, bad compressor, expansion valve, drier) the first time then you have to remove that R12 since your not really supposed to vent it to atmosphere. The second reason for pulling a vacuum is to test for leaks. At $9 per can that can get expensive if it leaks out.
If you replaced any part in the AC go in the service manual. There is a list of amounts of oil by part replaced that you should add back to the system. A new or rebuilt compressor comes shipped with oil. If you are converting an old system, most of the oil is likely leaked out. Some oil is always present in the system.
Mineral oil is not mixable with R134a and the old oil contains traces of R12 refrigerant which will react with R134a. Removing the compressor and draining the oil(usually about a 1/6 cup) helps. It won't kill a system not to do it but the oils PAG(polyalkyl glycol) and (POE) Poly Ester are displaced by the old oil. Some compressors can starve for oil because of this, even then oil is added.
The recommended procedure is to use brake parts cleaner (not carburetor cleaner and not the environmentally friendly brake parts cleaner), to blow into the lines, then use an air compressor to blow through each section. Put your thumb on the the exit, let it build pressure then release and let it blow out. This serves several purposes. It removes most, if not all residual oils, contaminants, and any metal particles if you had a blown compressor. Second, the brake parts cleaner evaporates quickly leaving zero residue. The particular section you want to do this with is the condenser and lines. I would stay away from doing the evaporator unless your changing the expansion valve.
Watch your paint, wear safety glasses, brake parts cleaner is dangerous. Oh, and its flammable with an octane rating of like 110.
Blowing out your lines is optional. Shops use a liquid flush and usually only if your looking.
Any section you blow out add back the proper amount of oil. Try not to exceed the maximum per the shop manual. The total for the compressor is "the" total amount for the system, not total added in addition to the other parts. when you blow out a part you are subtracting from the total the compressor came shipped with, which eventually gets distributed throughout the system.
You can pour oil right in the front fitting, then let it settle.
Adding refrigerant is simple, the low side line is on the top of the compressor. Do not shake nor turn the can over. You do not want liquid refrigerant, only gaseous (think hydraulic lock in a piston). Screw it down, make sure the valve is closed, puncture and open the valve. The compressor will not cut on until the system has over 30psi. Once the first can is opened and refrigerant is flowing(the can ices) the system will be at about 40 to 50 psi. Without undoing the connection start the car and set the idle at about 1500RPM. Kick on the AC(green). The compressor will start lowering the low side pressure(to approx 30PSI) allowing some of the remaining refrigerant to be pulled from the can. If you want to squeeze a little more out of the can, put the can in a pan of almost hot water(that will get you to 60PSI+ on the first can). Remember though that the running AC belt is right under the fitting.
Close the valve, undo and vent the can (watch out). Put on a second can and repeat the process. By this time you should be starting to feel the cooling. Part of a third can should have it cold. One line going in the back of the firewall should be cold, the other hot.
If you have a temp meter you can keep an eye on the temperature. The coldest you should be able to get it is 38 degrees. In reality I was only able to get as low as 54 degrees. As many times as I had to break it loose I may have tiny amount of water in the system or a bit too much oil, or a bit of air.
Its better to go have a shop charge it initially. A decent vacuum pump costs around $200. The harbor freight air compressor driven venturi pumps are not adequate for pulling a high enough vacuum to lower the boiling point of water to your climate temperatures. Proper gauges run about $75 plus miscellaneous fittings. There is also some who believe the larger 30lb canisters are better to charge from since you don't introduce the tiny amount of air and moisture from switching cans.
Be weary of some of the end all be all replacement kits. Be weary of the "hybrid" mixes that magically allow existing R12 to work with R134a. They are not considered compatible refrigerants. It depends on how good of shape your existing system is in when you do the conversion. Just changing to R134a it is recommended O-rings be replaced since R134a is a smaller molecule.
