... mixing/mastering. ...
That's just it. Proper mixing and proper mastering.
The thing you will want to learn is what YOU want to apply to your tracks because there is no ONE way to get a clean mix. So even if we say "remove low-mid frequencies" (which some call the muddy frequency), and you do that, but this does not apply to your genre, style nor intent of the track, then your mix will essentially not sound the way you want it.
Conclusion, if you want to know how then you have to study each element one at a time to know how it interacts with another element in your mix, if it complements and adds value or if it's destructive and takes away from what you want to hear, and how you want it to be heard.
Tips:
You should first get acquainted with your gear, and know how it works. Find tutorials for your DAW in specific, from beginner to pro. Know it inside out like the palm of your hand. When you're able to do this, you will realize that no matter what daw teaches mixing and mastering techniques, you will be able to apply it to your DAW. You will then learn it's limitations or where it excels compared to the other DAWs. When you're able to cross reference the knowledge from one DAW to yours then you now need to get more specific to how each element works and how it can be used. For example, instead of eq-ing a certain freq because a tutorial told you to do so and it's a habbit, know why you're eq-ing, the effect of the eq, what tone or shape the eq adds. Now do this for compression, stereo field, reverb, delay etc. Then you can mix to YOUR liking and intent.
If you're motivated then it won't take you too long. But first learn your own DAW.
Jay_