Hi -
I know this is a dumb question, but I can't find any tutorials explaining this.   When you're going to master a song, you have to export then import the entire mix.  So after you export, what exactly do you do to import?  Do I go into Sonar, click on New Project, then import the entire mix into that?
 
Thanks,
Greg

Take your exported, stereo mix (preferably a 24 bit export with a peak of no more than -3db) 
And simply drag it in to sonar. its as easy as that. you can import it if you'd like using the File> Audio> Import.
But I typically just grab the file from Windows Explorer (the place where I saved it) and Click/Drag it right on to the timeline. it will automatically create an Audio track.
 
This is a good thread on Mastering levels:
http://forum.cakewalk.com/FindPost/3524435
 
Also find a nice flow/chain of plug in FX for your mastering chain.
I use meters in the Master Bus and plug ins in the Track Bus 
 
My Mastering Chain consists of:
1-The SSL Bus compressor first.
2- then sometimes followed by the CA-2A for Analog coloring= a 1 to 1.5 db push / very subtle.
3- Linear EQ to drop off/filter APPROX 30HZ and under.
4- Then the L3.
5- Followed by the L2.
6- Some  more light Analog coloring.
7-The Secret sauce.

Here is what I do personally. Since every part is a group channel once mixed down, I export them separately into a new project. Then any slight tweaks or edits like adding an overall filter sweep on a part can be easily added without too much fuss.
 
So I have working independent stems thus removing any extra processing requirements which means I can use higher-quality plug-ins on them without worrying about any glitches. Reducing the audio path, so it's easier to concentrate on the one thing.
 
If you have a bit of whitespace (silent parts) between each stem, cut out those parts, make them clips and trim to save on disk space. 
 
Some people / mastering engineers prefer the isolated stems without equalisation or compression so it's a good idea to keep all this until the final render IMHO. Any processing you do at this stage should be minimal and not exaggerated to cover up mistakes.
 
It's not a dumb question BTW because asking for advice is healthier than guessing you around things.  The only similar thing about our systems is the choice of external sound device.
 
I just wish I could finish a project.
 

chuckebaby
Take your exported, stereo mix (preferably a 24 bit export with a peak of no more than -3db) 
And simply drag it in to sonar. its as easy as that. you can import it if you'd like using the File> Audio> Import.
But I typically just grab the file from Windows Explorer (the place where I saved it) and Click/Drag it right on to the timeline. it will automatically create an Audio track.

My brain likes simplicity.  When you say to "drag it into Sonar," do you mean into the existing song, or into a New Project?
 
As for Windows Explorer, I exported it to wherever Sonar exported to.  Does it automatically export it to Windows Explorer?  This all seems a bit complicated and is a bit over my head.  Are there any tutorials for this?  
 
Thanks,
Greg
 

Put more simply then.
 
In Windows Explorer, outside your current project folder create a stem folder. Or even on another drive if that's an option.
 
Export each group channel like your drums, keys etc. to their own stereo track in the newly created stems folder.
You can choose to do this at the export stage by selecting to export these as individual busses. You should only need to go through this process once.  Bypass any limiters and send effects you may be using as you don't any unwanted sounds in your individual stems.
 
Create a new project import the stems as suggested above. Remove any unwanted silent parts.  Maybe my ways are unique. I will try to find a tutorial for you.
 

In retrospect, my way is probably more complicated. If you did it with just a stereo track only I would still make sure my mastering chain was bypassed and reapplied in another project.
 

So here's a silly question. Why can't you master your master bus? Reason I ask is I've used lursen mastering plugin in my master bus and although I don't have as much control have not been displeased either. In theory couldn't you use the master bus and add your eq, l2, l3 etc... to achieve the same results?

highlandermak
So here's a silly question. Why can't you master your master bus? Reason I ask is I've used lursen mastering plugin in my master bus and although I don't have as much control have not been displeased either. In theory couldn't you use the master bus and add your eq, l2, l3 etc... to achieve the same results?

Yes of course you can.
 
I do this all the time for a quick rough & ready master with a chain of T-Racks modules

The whole concept of "mastering" is way overblown all over the interweb. Especially when referring to 1 song.
Mastering=finishing. Do it however you like.
 
If it's part of a collection, that's maybe a different subject.

interpolated
Put more simply then.
 
In Windows Explorer, outside your current project folder create a stem folder. Or even on another drive if that's an option.
 
Export each group channel like your drums, keys etc. to their own stereo track in the newly created stems folder.
You can choose to do this at the export stage by selecting to export these as individual busses. You should only need to go through this process once.  Bypass any limiters and send effects you may be using as you don't any unwanted sounds in your individual stems.
 
Create a new project import the stems as suggested above. Remove any unwanted silent parts.  Maybe my ways are unique. I will try to find a tutorial for you.
 

I believe that mastering is normally when you are working with a song as a single stereo track, following mixdown. 
 
What you describe sounds like the mixing process, working with sub-mixes, etc.
 
The simple answer to the OP's question is to export your final mix as a stereo wav file to a folder somewhere on your computer.  Then open that file up by dragging it into a new project in Sonar.  The wav should then open as a new stereo track in Sonar, so you are now ready to do your mastering!