servo tester

I'm a newbie. Just got my Crafty kit. I'm just wondering where to connect a battery to the tester. I saw that in a video here, so the tester needs a source of power. Not sure which prong to plug into, the S4? What type or size battery to use? Thanks!

Just wanting to center up the servo's w/o having to drag out the TX and hook up all the electronics......you know the drill. Any recommendations from the crowd on a servo tester? Thanks