I am system ignorant

I can't give you a step by step, because I don't know myself. I set up mine years ago and haven't had to mess with it. Having said that...
 
Since you are on Windows 10, it is likely the drivers that came with your Focusrite are not the latest (for Windows 10). I suggest going to their site and making sure you download the latest. I do not use Focusrite so I don't know the status but I would download new drivers them even if they were in beta (I.E. not released yet).
 
Once the latest drivers are installed, verify that the tools that come with your interface can communicate with the interface. If they can't, it is likely Sonar can't either. Troubleshoot that first. You might need to call Focusrite and/or visit their forums.
 
Once all that is squared away, open Sonar. Close the splash (or start screen) and go to Edit->preferences. The first item in the dialog (on the left hand side) says Devices. There should be a list of Input and Output devices. You should see inputs and outputs listed for your Focusrite device. Check the ones you will be using (I.E. Analog in 1, etc...).
 
I could be doing this out of order but on Playback and Recording, select ASIO (I assume Focusrite supplies an ASIO driver).
 
I think if you get this far, you are just about there apart from other customization you want to do.
 
If you are new to Sonar, please go to the CakeTV part of the Cakewalk website (Home page->Community->CakeTV. Start watching some of the beginner videos. They help.
 
Post back here to let us know how you are getting along. There are many helpful people here who know a lot more than myself.
 
Good luck!

maximumpower has it right, there is no point trying to set up everything at the same time. First make sure you are using the proper driver for your hardware. Not every piece of hardware is going to need purpose built Win 10 drivers. Focusrite reports that your unit works using: 
http://us.focusrite.com/answerbase/windows-10-compatibility
Scarlett 8i6, Scarlett 18i6, Scarlett 6i6, Scarlett 18i8, Scarlett 18i20
Works with Windows 10? YES - Using Scarlett Mix Control release 1.8
 
Plan on spending a fair amount of time (hours) reading the manual for the audio interface, and going over everything until you are sure you know what is supposed to happen when you do whatever you are trying. There are plenty of opportunities to send a signal to nowhere with the software mixer. This is definitely not plug and play. It is study and learn, and work and eventually you may have it become second nature and begin to play. You will waste a lot less time by approaching this as a tedious but necessary exercise, than by shortcutting it to the simplest understanding that will make noise. When you are certain you understand the workings of the Scarlett and it is functioning as you expect, then getting it connected to Sonar is relatively easy. If you are not comfortable with Sonar, then you can plan to spend some real time (weeks) getting to know the program. 
 
One piece of advice in the beginning is to disable or remove any other sound systems (soundblaster, on board audio chips etc.). It is hard enough to figure out how to make one work, without worrying about the others interfering with each other. If you need to use those other systems in the future (you probably will not) you can try to add them back again after you have your primary system working.

Can you tell me if you were able to use the SB SXR's ASIO driver with Platinum under Windows10? I am currently using an older X-Fi Fatality and cannot use the ASIO driver. Don't know what sound card or interface to buy. If it works I'd prefer the SB SXR over the Focusrite because of the optical I/O's.
I hope your problem from September was solved. Thanks for any advice.
Devin