3 ways:

Lower back: Lie down and extend your arms out to the side ("T" shape).
Roll your right leg over your body to the left, keeping your right hand on the ground as best you can and try to touch your right knee to the ground on the left side of your body. Roll left leg over to the right, keeping left hand down.

Upper back: requires helper that can lift you, and reach around you.

Stand straight and cross your arms in front of your chest, making an "X", hands to opposite shoulders. Have your helper stand behind you, reach around with both arms in a bear hug. Then they pick you up. Causes your spine to 'stretch' & arch backward a bit, I guess.
Gets great cracks from midway on up. If your helper is not tall enough,
have them stand on the first step of a staircase.

Upper back / no helper. Sit on the floor, legs together. Begin to lean back as if to lie down. Pause when the middle of your back is touching the floor; kind of 1/2 way into a crunch. Then just drop the rest of the way to the floor.

You left out "twisting" them…

This works on the middle knuckle.
"Grab" a finger: put your left index finger in the space between your right thumb and the inside of your hand. (the 'crotch' where your thumb connects") grab hold of it tightly with your thumb - just the tip of your finger showing. Now quickly twist your left hand towards your body...snap!!!

I get a 'snap' out of the Big Toe on my right foot…continually. I can do it 50 times in a row and just by moving it up and down without touching it ... so I don't think it really counts as "cracking".

I am bummed that I am no longer able to crack my neck

Back in high school, I could just stand straight up, and just drop my head
to the left and then to the right. I would get a drumroll of cracks each way. Grossed out everyone within earshot.

I think it was due to my slight frame (at the time). I was 6'3" and about 165 lbs.. My neck was "like a stack of dimes". I now weigh…. MORE than that...!!! I think my neck is too thick now.

Weird thing: My 17 year-old son... 6'2", 155 lbs....and he's taken over the family business!!!! Cracks his neck the same way dad used to, with the same results..!!! So proud.!!

Lie flat on your back, with your arms extended from your shoulders (think "T" shape). Take your right leg and roll over to the left, keeping your right arm down to the ground as much as possible… Repeat in the other direction.

This usually gets 3-4 of the lower vertabrae to crack for me.