The angle of the racket is obviously important but there's so many things... It's really impossible to 'teach' or explain. You just have to practice a lot.
For example, a few of the things that make a huge difference but are very hard to explain:
1) How exactly to hold the racket (depends on the size of your hands, the length of your arms, the size and shape of the handle, what you find physiologically comfortable or uncomfortable.. It's very idiosyncratic but it has the biggest effect on your serves really.)
2) How tight you grip the racket (also depends on how you hold the racket, because fingers apply pressure in different spots, but generally a tight grip at contact is important for generating high spin)
3) The behavior of your personal rubber (my tacky, hard sponge battle II behaves radically different than my soft, non tacky AK47 Blue on the backhand. It grabs the ball and imparts more directional momentum, so you have to be very careful to really generate a lot of spin on the serve or it'll all turn into forward momentum and you'll get high/net/out balls)
4) Your throw (higher throw can make more spin, if you want a really strong backspin serve, but too much can be very hard to control consistently).
5) Where you hit the ball in relation to your body (a bit behind, right in front, close or far from the body?)
In short there are a ton of variables which are unique to you and so only a lot of practice will do it, but here's the best tutorial:
https://www.youtube.com/watch?v=sjm9PvelFnA
Also notice the big difference between real backspin serves and so-so backspin serves: The real backspin serve is fast. The ball is fast, and it jumps back to the net fast. It can be fast because it has such a shitton of backspin on it. Look at ma lins serve at the beginning of the video. So set your goal not just to have a lot of backspin, but a lot of backspin and speed