Serving is most important and least practised. Players need to be able to exploit opponents’ weaknesses, and to do that one must first find out what those weaknesses are. In terms of the serve, this means that you must be able to serve the shuttle to the exact place you require. This means practise.
Practise a stock, reliable low serve to the T. Your fallback, steady serve.
Practise serving to each of the 4 corners of the service area.
Make a target in the service area, and then practise hitting it. Move it, then hit it again.
You need backhand and forehand serve. You need to know the rules regarding serve.
Fundamentals:
Backhand serve – thumb grip, racquet held in the fingers. Push the shuttle, racquet face almost parallel to the net (ie almost vertical), to ensure a flat trajectory. Move the wrist as little as possible. Nice relaxed grip. When you go to do the flick serve, simply squeeze the racquet grip just before you hit the shuttle.
Forehand serve - start with the racquet behind you, ensure you give yourself plenty of room for a nice smooth swing. As the racquet swings forward towards the shuttle, allow your weight to shift from your back foot to your front foot. Just before impact, whip the racquet through, turning your hand over (or your wrist over, or your forearm, it’s all the same).
Remember what you’re aiming to do with every serve. And practise.