OK, any half decent sparks would know, but anyway, a potted guide.
You will need:-
A multimeter (or at least a battery and test lamp)
A long wire that will reach from your breaker board to the appliance/outlet you want to protect.
What to do:-
Mains OFF
Connect your long lead to the neutral at the appliance.
Disconnect all the neutrals at the breaker board.
Identify which neutral has continuity with your long lead.
Repeat as necessary for each neutral you want to identify.
Put the neutrals in the correct RCBOs
Reconnect the other neutrals and the neutral tails from the RCBO to the neutral bar
Turn OFF all the breakers.
Turn ON the mains
Turn on your RCBOs (with luck they'll stay on)
Turn on the other breakers one at a time - hopefully the RCBOs will stay on (see below if they don't)
If turning on a breaker trips an RCBO there is a shared neutral, the easiest fix is to move the other circuit to the RCBO too. Finding the share is, er, fun.