Start in Settings, which you can access by clicking the Navigation button, then clicking Settings:

In Settings, click Audio and find your DAC. You should see a big "Enable" button next to it, like I'm seeing here for my DragonFly:

Click Enable, then name your DAC whatever you want. Next, click the little gear icon to access the settings for your DAC. Roon knows about many devices, and will automatically select preferred settings and apply the correct device icon.

If your Roon Tested USB DAC is not automatically identified, you can manually identify it using our "Find Your Device" picker.

A lot of the other settings will depend on what your DAC supports, what kind of content you listen to, and your own personal preferences. If you're unsure how to configure your DAC, feel free to create a topic in our Support section of the Community Forum. Finally, choose your DAC using Roon's Zone Selector, which is next to Volume along the bottom of the application.