The backband is a very important piece of equipment as not only does it create a connection point between your back and the boat. It also provides support to your lower back helping prevent injury. Different kayaks have different system however the basic concept is the same. You need you back band to sit against your lower back providing support without being overly tight. You still need to be engaging your abs and back muscles during paddling not just relying on the back band for support.
In a Jackson Kayak the proper way to adjust the back band is to grab each rope while sitting in the seat, lean forward and pull straight forward with both hands until the backband makes contact to your back, lean back against the backband and it should lock immediately into the cleats, you are done! To release, pull on one or both of the ropes straight out away from the cleat to release completely.
On other kayaks use a similar technique but adjust the rachet systems or buckle at the back of the back band to find the correct fit. The backbend should allow a full loose to tight adjustment in one motion, and also a full release in one motion.