Vin >------------LM7805--------silicon 4007 diode-------V+5out--->usb power pin
_______________|________________________________________
_______________|_________________________________________
_______________|___________________________________________
Ground >-----------------------------------------------VusbGround--->usb ground pin
note:
i'm not moving to this yet, cause i have a lot to do, but consider using a to220 LM7805 (not the tiny to92 package) just without capacitors and heatsink, cause it will protect battery from shorting, has thermal shutdown if overloaded and generally it is very stable and cheap component
of course you do it at your own
RISK