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)
The funny thing is, it's a lot easier to understand music theory than it is to understand how music theory will help your playing. It follows that spending time understanding the theory is time better spent than time trying to understand how it's helping.

When you're with the band, just put theory out of your mind completely. If you have to think about it, then you're not ready to use it anyway, and it just clogs up what should be an enjoyable an creative endeavour.

Remember that music theory comes from how music is practised, not the other way round. It's far more valuable to notice that C, E and G sound great over C major chords, and that adding a B to that sounds markedly different to adding a Bb, even though they both work (depending on the context). The fact that you're then using C Ionian or C Mixolydian (which differ by one note, namely B/Bb) is only a way to describe what your ears have worked out already.

Play when you're playing, study theory when you're studying, and trust that they will indeed converge and enrich each other.