It is very easy, locate the low pressure port on the Aluminum tube coming off the evaporator near the firewall, it has a cap on it. It is on the passenger side.
Buy one of the Tall R134 cans with the built in gauge at the parts store, should be $20-24 dollars. Will be marked max cool or EZcool or some other brand. Read the instructions on the can on how to pierce the can so it is ready to go, the one I used this weekend had a trigger and I liked that verses a valve. Turn the AC on max and connect the can to the low port (it is a quick connect snap coupling, it will not snap on the high port so don't worry. The high port is up on the compressor. A short time after opening the valve or trigger on the can it should start kicking the compressor on. Read the gauge after you let off the trigger or valve and that is the real pressure, it will spike higher when you are pulling the trigger. Depending on how low it may take several minutes to fill up to the full point. I filled mine to about 5 pounds below the full mark on the gauge. I am not sure how accurate those gauges are so I was being conservative. Anybody have anything else to add?
Oh, wear safety glasses, and don't leave the freon can in a hot closed car, theres a good story every time.
2004 Wrangler Rubicon