Lower the individual channel trim, fader the and main mixer gain controls to their minimum to avoid any loud noise when connecting the outboard equipment to the insert jack.
Lower the gain and effect level on the outboard signal processor.
Look for 1/4 inch phone jacks labeled "Insert" on the rear I/O panel of the Yamaha mixer. It is usually located near the microphone XLR balanced input jack for the individual mixer channel.
The "tip" portion of the insert jack sends the channel's audio signal to the outboard equipment and the "ring" portion receives the signal processed by it.
Plug the TRS stereo plug of the Y cord into the insert jack for the desired channel on the rear I/O panel of the Yamaha mixer.
Plug one of the monaural plugs into the input jack of the outboard unit and plug the other into the mono out jack of the outboard unit.
Slowly raise the level on the channel trim and fader controls and the master console volume until a signal is heard from the microphone or instrument connected to that channel.
Raise the input and effects level on the outboard until a signal indication is observed on its level meter or the processed signal is audible.
Swap the connections of the Y cord monaural plugs on the outboard device if no processed signal is evident.