If you want to make sure it holds a vacuum before charging, you have to go to a shop.
I didn't do that last summer, just charged it...it worked for a little while and helped me find my leak.
After replacing parts I went to a shop, had them pull a vacuum and then charge it.
You charge it on the low pressure line which is on the metal line at the top of the engine bay on the passenger side. Little black cap over the port.
Moss Green 95 5.2l zj- 3in lift (2" BB + Upcountry coils), 255 75 R17 BFG M/T on JK Moabs, Durango Airhat, Mopar ECU, Mopar Tire Carrier