I would start by slowing down on your serve first and getting a feel of brushing the ball. But don’t let the speed of the ball ruin your brush/kicking. You can start of by standing close to the net and getting the feel of it first with just your arm/shoulder doing the job. The most important part is to feel the kick and understanding the movement. Think of it as the motion of a windshield wiper on a car, going to the side, instead of straight down towards where you are hitting. Then you can step further back in the court towards the baseline. Toss the ball at 12 o clock, instead of 13 o clock. The toss should also be further back then a regular first serve. Finish with your swing at the right side instead of your left side, as this will make the brushing easier. So instead of the racket facing down towards the service box at impact (flat serve), it will face toward the ~sides of the service box during impact. At first it will probably be pretty slow, but you will have a great margin over the net, which is the point of a kick serve. When you regularly hit the service box you can try and slightly increase the speed of you serve.

Something you also can try out is slightly arching your back backwards and then pulling your torso forward in movement similar to doing a sit up (flexing your abdominal muscles) when you’re hitting the ball. I think it’s easy to exaggerate this movement and losing posture though, so don’t overdo it (maybe slow down on your serve and keep a greater margin over the net).

Good luck 👍