How do you gas up ac?

If you mean to "gas up AC" to refill with freon or R-134a for newer vehicles then yes you can, and it's very simple.

First hit your local auto supply chain. Purchase a recharge system, typically these come with an 18oz can of R-134a refrigerant, a hose with connector and a pressure gauge. They typically cost around 26.00 and allow for you to purchase refill containers for multiple uses as one can may not do the "job". Follow the directions on the the can, and find the LOW AC line. (all of these products have a connector which will only fit on the LOW side line) and many of the caps on the AC line will have an "L" for low or an "H" for high.

Next, turn your vehicle on with the AC button depressed, and on Max High. Make sure that your AC compressor is actually engaged. Most new compressors have a cut off switch, which prevents your compressor from burning up if the freon level is too low. A quick way to check this is when identifying your compressor, make sure that the clutch is indeed engaged and TURNING. If not, you will need to "jump" the compressor. Your local store will help you with this if indeed the compressor has shut itself down to prevent burn out.

Now that you have your car running, and have verified your compressor is indeed functioning, all you need to do is hook up the recharge kit to the LOW line. Check the gauge and see how much freon you actually need. Disconnect the line and screw on the can (if you have a trigger type refill kit). Screw on the can to pierce it, shake well and reconnect the hose. Pull the trigger while agitating the can from the 12 o'clock position to the 3 o'clock position. Stop occasionally to check the pressure gauge. Once you are in the proper "zone" according to the directions on the can, check your AC temperature inside your vehicle. You should be good to go after one can, or you may need to add another one.

This is an inexpensive way to recharge your AC. However, if you find that the following day or so, you begin to notice a decrease in pressure or cold air, then you have a leak and should seek professional advise.