TL;DR? use DC-DC converters to get the voltage you want/need, and don't skimp on the amps! Multiple converters can/should be wired in parallel off the battery "mains", as if you were plugging multiple household appliances into a wall outlet. If you are using multiple batteries to get more amps, avoid connecting them to one another, as they will try and charge/drain one another.
I've been getting a lot of questions about how to power arduinos and motors for robot projects so I put this together quickly. I hope to make a shorter version, but I haven't posted in a while, so I did this quick.
The UBECs are from www.hobbyking.com
other DC converters from amazon and ebay