I can't even believe it.. I disagree with Matt.

If you have the right equipment (ac manifold gauge and vacuum pump) you can do it yourself. That said I don't own those tools so I've been going back and forth on whether to buy them or pay someone. I just don't trust shops, so I'm more inclined to buy the tools and handle it myself. The only real question mark is how much oil is still in the system, from everything I've read the pros even guess at that though since they aren't pulling all the oil from the compressor before refilling the system.

The nice part about owning the tools and doing it yourself is that you can put the vacuum on the system and verify that you actually don't have any leaks, I expect a shop wouldn't test for a good sealed system for more than an hour or so since they need to move vehicles in and out so quickly.

As far as the ports go there is a low side (before the compressor) and a high side (after the compressor) and you fill into the low side. You put enough refrigerant in to meet the psi level that is appropriate for the external temp/humidity. The low/high side have different size connectors so you can't really mix them up when you have the right equipment to plug into them.