Originally Posted by zeke

If you just want to charge it, it is pretty straightforward. Can tapper, gauge and hose with the proper fitting (R12 I assume). Pretty cheap on fleabay
Get some refrigerant - if it's R12, you will have to get a license - some searching here will lead you to an online certification program.
Then tap the can and purge the lines by opening the valve in your gauge until the line is filled (gauge and hose) then connect the hose to the low pressure fitting (right in front of valve cover on w123 models). Start the car and turn the AC to max then open the valve in your gauge and add the proper amount of refrigerant. Low pressure should be 35ish? If you are really completely out of refrigerant, you may need to jumper the pressure switch on the drier to engage the compressor by running a wire between the 2 leads.

BUT, I think you should really be asking why you need a charge in the first place. refrigerant doesn't get "used up" - it leaks out, and if you have a leak, you will be spewing refrigerant all over town!

good luck!