Question: I am an amateur bass guitar player. At some point I decided to do something about my lack of theory knowledge.

Now I know a few basic scales and can read sheet music in simple keys (from zero to one to two flats/sharps). I understand in general how chords are built on degrees of the scale and know some basic ones. I understand what V/vii etc chord means. I know where notes on the fretboard are located.

But when it comes to real practice with a band, I'm completely lost. I can't seem to apply this knowledge in the process and reduce to catching roots by ear and playing cliches which often don't match the key and chord, and as I discover it I start alternating them nearly randomly (with the scale in mind if I'm happy enough to see it on the fretboard) until it sounds OK. 

**How do I bridge the gap between the theory and practice and start using the knowledge**?

Answer: 
@GeneralNuisance gets right to the point: music theory is a set of tools to describe music. But, would suggest a different emphasis on how to apply the theory in practice. 

I say **first analyze the music you play and listen to**, and become skilled at identifying musical elements. Analyze the connection between the musical elements you can identify and the expressive effect they have. Be cautious about using theory to support the creative process to avoid theory becoming 'the rules of music.' The worst would be misapplying the theory you learn using isolated theory concepts in the wrong context. You might find something innovative... but you might also botch things up. I think it's better to take inspiration from your analysis of real music.

Also, don't mistake theory books for performance method or songwriting books. Theory books will tell **what** to name things. Theory will also explain **why** things work. Ex: the resolution of dissonance/tension gives a forward impulse to music. Method books will show you **how** to do things. Ex: play the chord root on the first beat of a bar in a walking bass line. Theory doesn't tell me to do that. A method book describing the style tells me. But, notice how we can only get the sentence in the method, by understanding the theory terms: chord, root, first beat, and bar. 

Theory can illuminate the **why?** in the walking bass example. The root is played on beat one, because it most clearly identifies the chord and helps reinforce the meter. An approach tone is often played on beat 4 (4/4 meter) because a non-chord tone resolving by step to beat one of the next bar give the music forward momentum. Now that we have some musical context we can try to get creative with the application of theory: if the root on beat one is stable, maybe a rest on beat one and hitting the root on beat two an agitated or crazy feel. Try it and then analyze the result.
I'm a big fan of Ariane Cap's book "Music Theory for the Bass Player".

[https://www.amazon.com/Music-Theory-Bass-Player-Hands/dp/0996727604](http://rads.stackoverflow.com/amzn/click/0996727604) 

She has a full course for it as well (which you can find on her website). But for self-study, the book is excellent.