Turn off the UB802. Connecting cables between audio devices should never be done while the devices are turned on. Manipulating cables while powered can result in feedback that can damage your equipment or hearing.
Connect the UB802's "FX Send" output to the effects unit, audio interface or other device to which you want to route the submix. In most cases, this will be an effects unit such as a reverb or chorus device. A standard audio cable will usually suffice, but some devices may require a special cable. Check the device's documentation for details.
Turn the UB802 and the device connected to its "FX Send" output on.
Test the channel strips on the UB802 that you want to submix and send. Only the four mono channels can be routed to the "FX Send." Speak into the microphone or play the instrument attached to each mono channel comprising the submix. Watch the decibel meter to ensure there is no clipping. Turn down the volume on any mono tracks that clip.
Turn up the knob labeled "FX" on each mono channel comprising the submix. This will route, or "send," the signal from those channels to the "FX Send" output. From there, the signal will travel to the device connected to that output.