I just went through that on my scooter project last week. The top port on the tank is the regular, the bottom is the reserve. On the back of the petcock, reserve is marked the other would be regular.
Engine vacuum keeps the “on” port open to provide fuel to flow from regular to the outlet. During prime fuel flows from reserve line to outlet without vacuum. it is in the manual there’s a picture of it but it’s hard to tell which hoses which because it’s all in black and white and the hoses cross each other