Impact point is very bottom of the ball with very thin contact with a FAST bat at impact. You get this by being LOOSE and adjusting grip, stance, and swing mechanics.
There are a couple schools of thought in growing your ability to slice the ball on serves...
1) the school that says at first, do not worry about thin contact now... it will come with more training and timing and growing later... Just like NIKE... Just DO IT!
This school of though believes you will gradually be able to put better and better thin contact on the ball and not worry right away about blade angles.
Myself, while I can see some of the logic, I am NOT from that school, at least NOT the AFTER the first couple days. I believe one should learn a very fast, very long, very spinny underspin serve as a bridge to this serve, but one need not dwell very long on that part, it is so easy to get such a fast, long, spinny underspin serve.
2) the school that says learn the mechanics and practice "proper" mechanics to include acceleration and point of impact on the ball and bat angle at impact (which are essentially same if ball is tossed vertical)
With this concept, the player first learns the basics, then goes out and tries to apply them. What I always see players do wrong is they start with the proper blade angle to make at impact, but on backswing, they change the blade angle (close it 30-45 degrees) and on the swing do NOT correct the bat angle enough, so the result is a serve made with too closed (or better said not open enough) bat angle at impact that results in a ball struck way longer than intended.
I believe one should not focus on the speed/acceleration of the bat at first, but on the grip, stance, swing and angles needed at impact, to focus on NOT changing blade angle during backswing, even if the swing and result of serve is predictable. The important thing is to practice impacting the ball at the right point and slowly building up the ability to make better acceleration/bat speed at impact. This has a lot more profit after the first few days of fail, yes, FAIL you will at first. Once you make a swing with wrong blade angle on swing, you basically gotta blow up the swing and start over. It is WAY better to do the serve at 1/4 bat speed with correct impact (directly below ball at 6 O'clock) that try too soon for blade speed and get the blade angle wrong always. You see results of making the serve short withing the first few days, even if the spin is kinda weak, you know that you CAN do the serve and keep it short, even if spin is not overwhelming.
When the player gets better stroke and is loose, the timing gets better. I always ask players wanting to improve this aspect of their serve to practice while standing or seated while awaiting matches at the club... How?? You practice grazing under the ball, make ball go out a meter on the wood floor, then have the ball come back to you. If your impact angle is below the ball exactly (and later a bit to the front of under) then you get better spin as you get better stroke bat speed at impact. This exercise gives the player good practice at impact timing, ball toss, timing in general, plus the art of making that loose wrist accelerate to a nice speed without a giant backswing. Progressing in this manner is important in my (crappy) view in that it makes the player get right what is more important first.
look, when playing better players, it is more important to make the serve short and low, so what if the spin is not yet 150 rpm, but 83? When the player makes his 34 rpm serve look like a 67 rpm serve, he did his job and did it well. It is just that later, you need to show the ability to generate extra heavy spin to make your spin variety greater and make it easier to have opponent mis-read the level of spin, and later, be ready to make offense outta that. That is the basis of successful serving. There are many players in the elite ranks who have 2-3 really good deceptive serves from same service position and keep opponent off balance and get what they want from serve time and time again, or at least limit the 2nd ball attack options, or even take away opponent confidence to attack successfully. Too many balls hit into net or sprayed everywhere can get on your nerves for sure, why not take advantage of that like pros, even we are amatures?