Hi Donna, please work through Anders' vibrato lesson:

https://www.guitartricks.com/lesson.php?input=20182&s_id=1589

He's using the ring finger with the index and middle fingers right behind on the string for support. Slowly bend the note up just a bit (enough to hear the pitch increase slightly) then bring back down and repeat to practice the motion. Once you build up your strength you can start to practice this motion faster and faster.

Let me know how it goes!

Mike