I'm willing to help you out but I'm not sure what it is you want to achieve.
(1) There's an option in the master block (master block is accessed via the edit/solo button) to route tracks (i.e. mixer channels) 1 to 8 to the eight rca connectors. No mixing. Direct out style. If I remember correctly, this is pre fader. Might not be what you want. (operation manual p.73, anyway)
(2) Or do you want to send a mix to a rca pair (say, the one labeled "Aux A") that is different from the mix that goes to the master rca pair? In this case:
- In the master block, you can set which Bus you send to the "Aux A" RCAs. By default, the FX1 bus is sent there, but you can change it. let's say you change it to the Aux3 bus being sent. (operation manual p.72) - Now in each mixer channel (1 to 18, not only 1 to 8 ) you can specify the amount of signal that is sent to the Aux3 Bus. Make sure to change "off" to "pre" or "post" fader. (operation manual p.41)
(Note that Aux3 is labeled simply Aux if you've got an extra effects card installed)