https://www.justinguitar.com/guitar-lessons/bending-technique-in-blues-bl-403
John to reiterate the thumb position, listen to what just says at the 4:00 mark.
I would also recommend you practice just doing the bend, without applying it to a lick.
For example, use Close’s “Smoke On The Water” routine, referenced here
so you can focus on just getting the action and the target tone/note right. When you have that down use it in some licks. 