Just find a small fuel tank on ebay or somehwere online that sells parts for pocket bikes and the small chinese atv bikes and such, figure out where you want to mount it like behind the seat possibly, go to an auto parts store like maybe napa, autozone etc.... and get a Y union so you can have one fuel line going to carb and one each going to each tank, I would also pickup an inline fuel shut off valve in the lawn mower section at the parts house or at tractor supply so you can keep the tanks isolated from each other until you need to close off one and use the other.
This project is an easy thing to do if you have some tools and some know how, but how exactly its done will be determined by your bike, where you want the tank, how big of a tank you want and where you plan to mount the auxiliary fuel tank, you'll have to figure all of that out.
Best wishes
Map