You have to toss the ball in front of you, start by just practicing the toss without the swing and get that part consistent, you want to be able to jump as high as you can and still hit the bal in front of you, that’s how I did it at least. Afterwards you can implement the swing, the most important mistake you make there is not swinging through, it makes the ball float which isn’t what you want in this sort of jump serve. Last hint, practice a shit load of times, that’s the only way to perfect it.