You don't.
I still had my hand on this one when it blew up as I plugged into the wall. What did I learn? Charge on a charging strip, with charging strip in OFF position, take hand off wall adapter/charger, then turn charging strip on. Also charge in safe place, lipo bag, etc. Unless you want a $150 electrician's bill like I got.
Also, I later closely inspected the wall wart. One of the "tines" was wobbly. See, all this was my own fault, but I know more now then I did then.
It's not a problem, none of it is a problem, if you practice safe vaping overall. This could have been avoided. It scared me though as I was plugging it in at the time, so my hand was ON the charger:
If you are using loose lithium batts, have a multimeter esp. if using a mechanical mod. Meter batts off charger, let them rest before using, meter them to make sure you aren't over discharging when vape starts to feel weak, etc. etc. etc.
All vaping problems are avoided educating oneself.
(electrician did test my charger, it was bad. Thankfully, my electrical system is top notch, the circuit breaers in house tripped as they should, etc. )