I have two ways i crack my knee. On is to stand up straight and twist my knees left and right. This usually works.
The second way i crack my knees is to pull my foot [ left or right], to the side of my body. I then pull upwards causing presure in the knee. This also usually works.
The first method that I usually use is to sit down at a chair and face forward. I then twist around and look the other way. You then should hear a cracking sound obviously meaning you cracked your back.
The second method i use to crack my back is to place both of my hands on each side of my spine, and push inward. You will fell pressure but you should then crack your back. You can put your hands on each side of your spin at any part. It works at all spots for me.