The simplest way is just to turn Hardware MIX on 1st, before you load the project. Inside the project, go to VST connections.
-Under INPUT tab, select the channel you want to use (N12 Direct Out 1 and 2, for ex.) Under OUTPUT make sure N12 Monitor or N12 Aux is selected, depending on your setup.
-On the Cubase mixer, just set the output routing at the top of VST channel to N12 1 and 2.
-N12: Make sure the ON light is on. Make sure either ST or REC is lit. Make sure STEREO buss is lit ON.
-Back to the Cubase mixer. If you want to route a send to the N12, create send or FX buss. Send VST channel to the send. Disconnect the Output routing from N12 1 and 2. Instead set the send or FX buss output to N12 1 and 2.
-Or send several VST channels to that group. Now you have a buss going into 1 and 2.
-Go back to VST connections. add + input 11/12.
-Change the group output to 11/12. Make sure 11/12 ON light is on, and ST or REC is lit. Now you have a STEREO buss (with no compression ...ahem
There's a blank SQUARE where 4 more knobs could FIT argh LOL just kidding George)
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The way I bounce my VSTs back... I patch the output from the back of the N12 into the RCA input of 9/10 (A.IN selected). So the output of 11/12 goes to Stereo. Stereo goes into 9/10. Use the Stereo Master fader into the GAIN of 9/10, and check the input of Cubase IN channel for overs. This sounds better than the standard REC buss IMO. Furthermore, I take that resulting audio mixdown. Send that BACK thru 11/12 Hardware mix. Add UAD compression and EQ sweetening, tad bit of N12 90 hz small boost, bit of high cut so it's not so "digital", limit it. Then Cubase Export. They are the loudest, clearest bounces I have ever made. In some cases room to go louder. I usually roll back on the 10 khz just a tad to let back on any harshness, and I'm sounding a lot better with this method. I actually haven't done a full hardware mix in awhile (with all cubase channels going to separate N12 channels) it's actually still too much effort for the way I work, and the 2 mixes I did that way came out slightly bright for the type of stuff I do.
BTW If you want to use Sweet Spot compression going back in with the above method, just patch L & R from Stereo out to mono channels accordingly. However the stereo image of the mix will fall flat. I have not figured out how to go back in using the N12 compression and keep the mix correct.