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: 
The first, and most important thing:

**Music theory is a set of tools to describe music.**
Theory does not dictate music, music dictates theory.

For example, a time signature on paper does not dictate how the tune will sound, the way the tune sounds will dictate how the time signature is written.

With this understanding comes the ability to apply music theory in a practical way. Music theory is not usually something you use while improvising, and it's not something you think about while performing, it's a language that you can use to describe what you did and to help understand *why* something sounds the way it does.

Instead of trying to learn music theory in a text book, I would learn it by use. Try stuff out. If it sounds good, find out why. It's all fine and good to know what a V/vii chord is, but you'll only use that knowledge practically if you need it for something. 

Think of it like a box of tools. If you learn what they're called, what they do, it still won't be practical until you try to build a chest of drawers and you realize you need a way to cut wood, or hammer nails. 

Of course, all of this is really easy to type up but harder for you to go out and do. As you go, try to identify different musical concepts that you see/hear. Almost as an aside, having access to a piano or keyboard will absolutely increase your understanding by leaps and bounds. 

When you learn a new music theory concept, make sure you learn the "because" of it. *Why* does putting a Bb in a key signature make it F major? Why does slapping a capo on a guitar change its key? This is when connections start forming.

So in short, to make music theory "practical," you must have a use for it first. Learn music theory to solve problems that you encounter. See some of the answers here: [How do we teach music theory well?](https://music.stackexchange.com/questions/50757/how-do-we-teach-music-theory-well)
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.