As a bass player, how do I go about learning music theory?

You don't

>>74998512
bump

>>74998512
Buy the real book, learn how to play the songs and then learn how to play the melodies of the songs

>>74998512
Study several different scales and the relation between each of them (e.g. the difference between a major and minor scale is the third note of the major scale being flattened.)

There's a book called "The Guitar Grimoire: Scales and Modes" that gives you pretty much every accepted scale from pentatonic to 8-toned. There are other books with the same title, but I've only picked up the scales and Modes one.

In addition to scales, when you listen to music, try and pick out parts of the song that create a polyrhythm. It's easiest with drums, where you can focus on every snare hit or kick. When you do this, try and get a feel for how it builds or supports the rest of the song.

Also, be diligent. If you really want to learn anything you have to be willing to "waste time" on whatever it is.