1.) Put your hand in “normal” or first position. Choose a finger to slide on the neck. Your middle finger is a good and easy one to start with.
2.) Slowly slide your hand back and forth on the neck of the violin. (From first position towards the bridge.)
3.) Gradually decrease the distance of your back-and-forth-slide
4.) When the distance becomes very small, plant your finger in it’s “home” or regular spot. Rock your chosen finger back and forth. You should still be going slow.
5.) Gradually speed up. You will need to do this whole exercise multiple times. you can slowly speed up how fast you slide and rock your hand/finger. Vibrato is not an overnight thing. It takes time and effort. thing exercise should help you learn vibrato, and if my instructions were unclear, there are youtube videos to teach you but I used this method and learned a good vibrato in no time.