I wrote this long reply, and it didn't get posted for some reason. Well, it was probably more than you wanted to know anyway.
I master right in Cubase right in the project. The PC is powerful enough enough that this works. I can, if I want, go tweak something in the mix that affects the mastering. If I'm making a CD, I use Wavelab, although no effects, just the organization and administration.
On my master bus, I first put a Cubase compressor, set for mild mastering compression. I tweak it to ensure that I don't compress by more than 2-3 decibels. This is the first place where the instruments interact dynamically. In fact, I do this during mixing. Gives it a little cohesiveness.
On the second slot, I put an amp simulator to get a little warm distortion. I am using TLS_Saturated Driver which is free.
Then I go to Ozone. I used to use the Cubase plugins for this. What I am trying to do is: 1) equalize the whole thing 2) stereo widening, and maybe harmonic excitation 3) loudness 4) dithering (because I'm recording at 24 bit).
For equalizing, you could just use the Cubase 4-way equalizer. The aim is to compare your mix to your favorite reference mixes. Those are your favorite albums/recordings in your genre. For the stuff I do, which is fusion-y jazz stuff, I find that the low mids are always muddy and need to be reduced, and the highs (7000) need to be raised a couple of dB. Probably a consequence of using guitars and keyboards.
For stereo widening and harmonic excitation, Cubase has some plugins that I never really got to work well, but I do like what you get in Ozone. This is to taste, and basically, when you like it, dial it back a notch to be conservative.
For loudness, there are two things. Multi-band compression brings the volume up by frequency band, and can really have an effect on the equalization. This is a difficult one, and you have to balance your tweaks here with what you've done in the equalization stage. Some would say just skip this and effect the changes elsewhere. Cubase has MBC, and I would just say it's difficult to understand and control. But I'd say that about Ozone too. I'm still learning.
The second loudness thing is a maximizer. Cubase has one, which I think works pretty well. The ones in Ozone are more sophisticated, I think. You really can't tell what Cubase is doing, other than what you hear, but you have a better shot at seeing/hearing what Ozone is doing. If your mix had interesting dynamics beforehand, you should be able to make it louder (almost as loud as your reference recordings) without killing those dynamics. If your recording was very flat from beginning to end, neither will help.
Dithering is the standard last process, and you can read about it on the internets. Cubase gives you a plugin. You should be recording at 24 bits.
To check the final recording, I listen on all systems available. All the headphones you own, all the cars you own, laptops you own, oh, also your monitors. Always comparing with the reference recordings you love so much. I sit in my car and take notes.
To hear my results, listen to my album in my signature.