Hi
The inverter will be fed by two wires, one from the red / live terminal of your domestic bank of battery's and one from the black / negative.
These will be big cables but there will be further big cable from the same battery bank to supply the fuse board and all other 12V appliances.
The shunt is fitted in series, that is in line, with the main black wire from the battery bank.
If there is more than one black wire they need to arranged so that only one cable leaves the battery bank - or you will only read the current going either to the inverter or the fuse board.
This one cable may need to be even bigger to take the current of the others.
I hope this makes it a bit clearer.
Alex