Three line : one line to the clunk --- this should clear the dead center of the back of the tank when held upright. This connects to the carb.
second line to the vent . The metal tube should just clear the top of the tank inside.
third line to a second clunk (if you plan on de-fueling) or just inside the tank if you don't. This is the line which is plugged after filling. Anything can leak but seldom is it the "third line". A 4/40 by .25" screw will work well (assuming medium size fuel line).

Two line : same as one and two above. Just omit the third line. To fuel/de-fuel unhook the line at the carb. If your installation wont allow it (cowl etc.) then use a DuBro filler valve or the equivalent.