Hey Josh,
You just use the one battery, it will plug in to your esc and this will in turn plug in to your receiver.... powering your servo's, when you get to the low voltage cut off your motor/power will decrease but you will still have power to your servo's. Depending on how many servo's you are using you may use an external bec.
There will be others on here with more experiance that will add more info.
Best of luck,
Reuben