You must toss the ball upward at least 16cm or 6 inches. (Rule 2.06.02) - Its about the height of the net.
A lot of players players don’t get the required height from their toss and will either hit the ball right out of their hand or do a quick drop-and-hit. This creates an unfair advantage because it takes more skill and focus to control a ball on a higher toss than a drop-and-hit where the ball is practically stationary in the air.
Your toss must be “near vertically upward”, not heavily sideways or backward. (Rule 2.06.02)
It’s common to see players throwing the ball backward (away from the table) to help generate more underspin, but try to avoid this practice. Your toss should be mainly vertical, with very little horizontal movement in any direction. It’s somewhat problematic that the rule is vague as it doesn’t give a definition of how straight your toss must be. Can you toss it at a 45° angle? Is that vertical enough? Referees will use their own discretion, but try not to dance on the fine line. My own tosses are generally about 15° from the vertical.
You must strike the ball as it is on its way down. (Rule 2.06.02)
You can’t hit the ball as it’s on the rise from your toss, it must have started falling before you make contact. It doesn’t have to drop all the way down to the point where you tossed it, but it has to be falling. So if you tossed it 20cm high, you can hit it when it drops even 1cm from the top of the toss.
The entire serve (from toss to first contact with racket) must start behind the end line and above the table surface. (Rule 2.06.04)
Some players will either start the toss with the ball already inside the table, or will strike the ball while the ball is over the table, which is illegal. Also, the ball height must always be above the table surface, so you can’t wind up your super-high toss by starting the toss below the table surface.
You cannot hide the ball with your body or arm during serve. (Rule 2.06.04 and 2.06.05)
According to the old rules, you could leave your free arm in front to obscure the view of the receiver so they can’t see what serve you’re doing, but that has changed. Now you must remove your free arm (and body) so that the receiver has full view of the ball throughout the entire serve process.