One of the easiest way's to start translating theory on the bass is to take a look at the various shapes a particular harmony might suggest on the fingerboard.

For example. Take a look at the finger diagram below. I have laid out an interval of a Major 3rd C to E.

Take a look at the shape this creates on the fingerboard. The E lies a string up and a fret over. Or, if you are using the Finger-per-fret technique with you middle finger on the C then it looks like One string up and a finger over.