Vibrato really needs to be demonstrated rather than described, otherwise your interpretation of the description might not be what was actually intended.
In basic terms, vibrato on the cello has to come from a rocking motion not just of the hand and respective finger, but from the forearm too. Make sure you keep your left arm out and up, rather than clamping it down on your side - the latter of which will make effective vibrato impossible. You also need to make sure your fingers are curved over the strings properly - stopping the strings with flat fingers will do no favours either.
Speak with your teacher and get them to demonstrate the vibrato action slowly, then immitate this with their guidance and practice it slowly to begin with. Don't panic if it's not sounding right immediately - vibrato is something that develops gradually as the strength develops in your hand, and fingers become more independent of each other.
Remember, vibrato is a way of adding expression to music, not a means to cover up dodgy intonation (although I've heard it used like that on ocassions
). In the syllabus it says that by Grade 5 the student should have developed "some skill in vibrato", so the examiner won't be expecting Yo Yo Ma, but do make the effort to try and do the vibrato as correctly as possible.