How to do it
1. Step back your dominant leg so that it will be behind your other leg
2. Jump / Raise your body upwards
3. Transfer your power from your legs -> torso -> arms -> wrist -> racket otherwise it wont be as strong as a normal smash
4. Hit the shuttle at your highest point of the jump for the shot to be more steep
5. Land and return at the middle

Tips
1. Step back from the approaching shuttle
2. Be Relaxed , being tense might make you miss the shuttle
3. Bend your knees 90 degrees while in the air , it makes you more flexible

I admit that i cant always hit the shuttle eveytime i perform a jump smash , but practice makes perfect.

Get a friend , go to a court and make sure to bring a lot of birdies.
Make him/her continously make high serves towards you.

I also need some help on timing
I hope you can help me as i helped you