On a small piece of perfboard, solder a 5V regulator. Attach three wires with female headers to the perfboard: one to the regulator input (far left pin), one to the regulator ground (center pin), and one to a free row on the perfboard (servo control).
Solder three header pins to the perfboard in a row: to the voltage regulator ground, output, and the servo control wire.
If you want to control more than just a servo, use a buck converter! It can deliver more current without heating up. It is wired similarly to the voltage regulator where the 12V battery source and ground hooks up to the input, and the header pins wire up to the output + and -, as well as the servo control as above.