Well briefly you would set up a new stereo output in vst connections....do not connect it to anything.
Then you will need to route your mix to this.....unfortunately you can't route the stereo out directly to it so you have to switch all the channels currently routed to stereo out to the new phantom out. (You can use shift alt to apply routing to all selected tracks)
If you have any master bus fx, you'll need to move these to the new output bus too.
Then you can just create a new Stereo track and set the new bus as it's input...arm record the track, hit record and let the song play through to the end.
i7 firstname.lastname@example.org, 12Gb, W7HP x64, Cubase 7.5.20 x64, Steinberg UR28M, Alphatrack, Wavelab Elements 7.2.1.