Samsung SPH-M390ZSWVMU - Manual (Page 9)

Get Started 2
 Insert the battery, contacts end first, and gently press the battery into place.
 Replace the battery compartment cover, making sure all the tabs are secure and
there are no gaps around the cover.
Note: When you insert the battery, your phone may power on automatically.
2. Plug the USB connector into the charger/accessory jack on the bottom of your phone.