Starter battery positive connects to the starter battery - normally on the input side of your DC-DC charger - somewhere between the starter battery positive and the DC-DC input terminal
Leisure battery positive connects to battery, or busbar or wherever your positive connections are - alternatively you can connect to the load output of an MPPT controller
Negative goes to the negative connection on the leisure battery, or as above, to the load output on the MPPT.
There's really not much more to it...