Here’s a starting position:
Stand facing straight ahead, arms down by sides, violin in left hand, bow in right, feet slightly apart. Relax (take three deep breaths. Hunch your shoulders and drop them – repeat three times).
Stay facing straight ahead – don’t move your head up or down or to left or right. Lift the violin/viola into position against your neck; put your left hand under the body of the violin/viola to support it.
Turn your head left and right. It should be completely free to move. Keep facing forward. Don’t turn to look at your left hand, or tilt your head to the left (you see this a LOT! Try it without the violin and feel what it does to your neck muscles!).
If you have the correct chin rest/shoulder rest setup (every person is different) you should now be able to tilt your head slightly forward so that the violin balances on your shoulder with a little support from the left hand. DO NOT CLAMP DOWN with your chin.
You need only grip with your chin – if ever – when sliding from a higher position to a lower one. Make sure you relax again after.
With the instrument balanced correctly, your left hand should now be able to move freely up and down the fingerboard, just giving a little support to the violin neck. If not, then your chin rest/shoulder rest setup is wrong. Go to a music shop and ask to try lots of combinations. Experiment until you can support the instrument without tension. Remember – head up, facing forward; shoulders relaxed.
Make sure your left wrist is down, not up (ie wrist bent AWAY from the neck, not towards).
Tension is the enemy. Start out with a bad position and you may never lose it.
I hope these notes will help you.
Enjoy playing your fiddle!