You put the switch to center or outboard and then they will automatically transfer fuel as soon as the internal fuel is below a certain value. Once that happens, the external fuel is pumped into the internal until they are full. Then the transfer stops until the internal fuel reaches the value again and the process repeats until the external tanks are empty.
For more information, you should consult the manual.
How to optimize it? Just keep the switch in the outboard or center position until the externals are empty.