Vacuum the system for 30 minutes with both ports open, then leave the system closed for at least 10 minutes after vacuuming and before charging. If the gauges hold a steady vacuum, its a good bet you will have no leaks. Not a foolproof leak check, but a decent one.
If you have a scale and a large tank(not the autozone beer can size ones), charge the entire system BEFORE YOU START THE ENGINE from the HIGH side(red fitting), or as much of the recommended charge as will flow into the system. This keeps any chance of liquid refrigerant from going straight to the compressor(liquid doesn't compress) on start up and causing a failure. Always close both fittings before starting the engine. If a full charge did not go into the system before starting, now you can finish the charge from the LOW(blue fitting) charge port with the engine running and AC on max cool. Adding a couple ounces of PAG oil into the receiver/dehydrator(dryer) before you do anything wont hurt either, especially doing engine swaps or conversions where you may not know how long its been since the ac worked. Should be a sticker under the hood showing how much refrigerant each system holds. It definitely aint rockect science, but like anything practice makes better. After a few vehicles, you will figure it out. Pressures vary
depending on ambient temp and humidity, air flow to the condenser. There are charts online to give you a base line on pressures, but if its sweating on the low psi side, hot on the high psi side, high psi around 175 to 200 just off idle, low psi 35 to 45, then you are real close or right on.
