Focus on the correct grip, early preparation, using your non-dominant hand for balance, maintaining the correct contact point behind your body, using your legs to generate power and height, and practice specific drills targeting these elements.
This lesson from coach Sven is very valuable if you want to improve your overhead game, specifically hitting the kick smash out of the court from further back. He covers essential tips on grip, preparation, and contact point, explaining why they are crucial for generating spin and power. The drills shown are practical and can definitely help you practice this difficult shot even by yourself. Learning this shot adds a great offensive weapon to your game.