I've injured two knuckles trying to get them to crack when they wouldn't. Eventually I broke the habit of cracking knuckles at all to prevent further injury.
For those who read about the doctor who popped one hand with no issue is only a sample size of one and he only popped the knuckles on that hand once a day. When I started trying to break the habit I was tracking and I'd often crack a single knuckle 30+ times a day and did for 20+ years.
Crack responsibly, unlike me, and you're probably fine.
It was more like I could tell it would pop but it just needed a like more pressure. Popping knuckles stretches the tendons and my guess is since I was going it so much I stretched them so far that I had to push my fingers too far to get them to pop.
Same, when coming out of surgery on anesthesia my nurse kept trying to remove a line from my hand and i wasn't having it. Had knuckles to crack. The habit for me is just that ingrained.
I kept track of every single knuckle pop, so if I popped one whole finger it would be 3 pops. I started by measuring my baseline and then setting goals. If I made my goal that day I could do something nice for myself like play video games or get desert with my family.
Took me almost 2 years.
And for 3 to 4 years after occasionally I'd have nightmares that I started popping them again. And over that same time period as winter would approach the desire to pop them got really intense.
Take your thumb and put it just above the joint(towards the top of your finger)
Now take your pointer finger and rest it under the first joint of your thumb. The tip of your pointer finger should just about be at the webbing between your thumb and other pointer finger. You should now have a finger on either side of the "thumb bone" and on opposite ends.
Gently apply pressure with your holding thumb downwards. Not the thumb that is being held. It should have about a half centimeter of wiggle room before it pops.
You can also pop the last joint in your thumb by cradling it similarly, titling it 90° from the rest of the thumb, and pulling away from the hand. This technique works on every finger tip.