1. Go Insert > Stereo Bus to create a “virtual aux bus” in Sonar, then name it.
2. Assign the bus out to your interface’s analog output. Patch the analog output to your processor’s input.
3. Right-click in the blank space of any track that you want feed into the newly created aux bus, then select Insert Send and the destination bus.
4. Enable the send, then adjust level, pre/post, and so on.
5. Go Insert > Audio Track to create an audio track that will receive the interface’s analog input.
6. Assign the track input to your audio interface’s analog input. Patch the signal processor’s output to this analog input.
7. Turn Echo on, otherwise you won’t hear the signal coming into the track.