The way I think of vibrato is that it's like two things: the start and finish of a position shift with nothing in between, and like shaking hands with the bass (ok, left handed, but still). It's that same sort of reflexive shaking, a bit like flicking water off your fingers.
For a start, practice vibrato on long notes, and scales. You should do scales without vibrato as well, because the vibrato will obscure the intonation a bit, but they help in getting the fingering transitions in the vibrato when you're starting it.
The tricky bit is to keep it under control, and use the appropriate amount. That is part of the dynamics and the shape of the musical line, and really a matter of practice and making a musical decision as to what is appropriate.