There’s a couple of different ways of doing this.
The first method is to use a circular service motion. Your service swing needs to have a downwards part and an upwards part. A pendulum serve is a good example of this. Then to generate different spins you should contact the ball at different times in the swing trajectory.
If you contact the ball when your bat is swinging down, you will generate some backspin. If you contact the ball when you bat is rising you will generate some topspin. To your opponent the service action looks the same. If your opponent is not concentrating, he/she will likely make many mistakes.
Another method is to simply change your bat angle at the very last moment. This is particularly effective when for varying backspin and no spin serves. For your backspin serve you should keep your bat angle more open to brush under the ball. For the no spin serve, you use exactly the same service motion, but just straighten the bat angle at the last moment. This will generate less spin. If your opponent does not read the ‘no spin’ serve, he/she will pop the ball up high or hit the ball off the end of the table.
Whichever method you use, I personally think it’s very useful to have a fast and whippy service motion. If your service motion is fast it is harder for your opponent to work out the contact point. You don’t even need your service motion to be exactly the same for different spins. You can have slightly different service motions for different spins and it can still be hard for your opponent to read the spin, as long as the motion is fast and whippy.