Adult self-taught violinist here. I've been learning for about three years.
I've developed a habit of allowing the left-hand thumb to protrude rather high above the fingerboard, so that the neck of the instrument rests close to the base of the thumb. From various YouTube instructional videos and observing good players, I understand that this is sub-optimal, particularly with regard to eventually developing a good vibrato. So I am trying to correct this, keeping the thumb lower down, with the end of the thumb touching the side or base of the instrument's neck. I understand that the thumb and index finger should form a quasi-circular arc, almost as if pinching something.
The problem I am having is that the side and base of the violin neck are insanely slippery and I am having trouble getting the tip of the thumb to stay in position when in contact with that area. It just slips around all over the place.
Any advice about how to deal with this issue would be appreciated.
Creative but probably bad ideas that I've had include using sandpaper to roughen the back of the violin neck and make it less slippery, or gluing something non-slippery e.g. a piece of velcro there. I realize also that some advice might be along the lines of "get some in-person lessons", however finding a suitable teacher where I'm located at the moment is hard, especially under pandemic conditions.