How can I charge a disposable vape pen?

If you are asking for the ejuice disposable pen the answer is here :

This is an incredibly late reply, but it's super easy to charge disposable vapes.

Take the bottom piece out gently, until you see the red and black wires connected to a metal dot or a metal tab. Strip a phone charger and hold the red charging wire on the red vape wire, and the black charging wire on the black vape wire. Put the wires on the metal tab or dot that the other ones connect to.

Do this at your own risk, however, as they are not meant to be charged.

If you are asking for the THC Disposable Vape Pen , it is too easy , almost all the disposable vape pen comes with charging port some are micro usb some are type c port . 

you can use your mobile phone charging cable to charge it .