How to measure positive, negative and inbetween currents
For the voltmeters you need to connect them between +12V and GND and -12V and GND.
The ampere meters you should connect them in series with the outgoing +12V and the connector and the same for the -12V and the outgoing connector. If the power supplies are separated ( missing information!) then you could also place the ampere meters in both GND lines and combine the output on the connector.

In the situation that you are only interested in the differential current in the GND connection then you connect the two GND connections before the outgoing terminal. The ampere meter should in that case come between the two ground connections and the outgoing terminal.Depending on the load the current could be sourcing or sinking. 

If you are using digital volt/amperemeters you should be carefull. For most units there is no galvanic separation between the supply and the measuring connection. In that case you might end up with separate supplies for each meter or at least for two of them. For the amperemeter in a common GND situation you need to select a type that can handle both positive and negative current. Not all types can.