"Set up the hand with the palm touching the violin. Make sure every finger moves from the base knuckle."
Touch the base knuckles of your left hand with your right hand to focus your attention on the correct finger action. The action of the 2nd finger lift and drop is usually correct so focus on this finger to start with.
To further develop a stable hand frame, hold the 4th finger down on the A-string and tap the other fingers on the D-string.
Make sure the 4th finger is sufficiently curved to give sufficient strength in the finger-fall and avoid collapsed knuckles. However, avoid exaggerating the arch of the fingers since this limits stretch and the hand may become tight.
Finger Action - Introduction