Looking for Highly stable 5v Regulator
Since you are battery powered, you probably want to use switching regulator for maximum battery efficiency.

The cheapest way is to buy a pre-made regulator module. '5V step down' will give you plenty of results, for example this one $1.70 with free shipping: http://www.ebay.com/itm/like/221744965891. It can take up to 24 volt input, so you can just connect your battery pack to the input.
The regulator above is using KIS-3R33S module. If you are limited in space, you can buy the module alone, and add two capacitors and two resistors.

If you want to make one yourself, I highly recommend TI 's "SIMPLE SWITCHER" series, such as LM2673. Here is an example schematics:



https://www.ti.com/product/LM2673/technicaldocuments

Note that if you going to make it on protoboard, you will want to keep the wires short and ground in one spot, as per regulator datasheet.