First you have to hit the ball accurately. While all shots require accuracy, backspin depends on the point of the ball is hit. If the ball is not hit on the face accurately, there will be no spin. Also, it is important to swing quickly. Swinging quickly and hitting the ball accurately is the key to control your spin.
Backspin is a difficult technique. The club has to hit the bottom of the ball accurately and therefore, the swing needs to be clean. Of course the face should not close. There should not be too much power involved at impact. Having too much power decreases accuracy. Swinging the ball after placing it slightly to the right of the center is recommended.
The most basic method for adding backspin is to open the club while swinging. Also, early cocking is crucial. I guess cocking is the easiest way to have backspin. However, I don’t use the backspin often. I only use them when necessary.