fret a note, and slightly move your finger either:

up and down - the note will go sharp and then back to pitch and sharp etc while you wobble

backwards and forwards (towards the bridge and then towards the headstock) - the note will go flat the to pitch then sharp then back to pitch then sharp etc

and then theres a mix of these 2, where you move the fretted note in a circle giving a slight different soudn

these are done pretty quickly and can really bring a fretted not to life

EDIT: well the link above to Vais site is where i learned all this stuff so, i think that might explain it a little better
when you do vibrato make sure you are first bending the note to pitch before doing the vibrato. that way you wont sound out of tune. and yuo might wanna use your wrist to help give you more force and smoothness.
Make it come from the wrist, like turning a door knob, not from the fingers.
He who joyfully marches to music in rank and file has already earned my contempt.
He has been given a large brain by mistake, since for him the spinal cord would fully suffice.

Remember: A prudent question is one half of wisdom.