4. Practice, practice, practice
Improving and applying theory requires effort. If you’re not going to practice, you’ll never get any better. And by “practicing,” I don’t mean running through songs.
If you want to learn a language, you can’t learn a bunch of phrases like: “Where’s the bathroom?” or “How much does this cost?” That will only help you in very few instances, but it won’t help you express yourself in that language. It’s the same thing with music practice. By practicing songs, you learn songs. By practicing the “boring” stuff like scales and chords, you create a vocabulary that helps you express yourself.
If you’re a guitarist, learn your scales and chords all over the neck. If someone tell you that they want to insert a Gmaj7, understand how that’s different than a G7.