Often I find it very difficult to time my jump exactly right, often jumping too late and then missing the shot entirely. What can I use / practice to ensure I time my jump correctly?
As Rory mentioned, practice will help - this technique is considered to be fairly advanced. Repetition is needed to perfect your technique.
If you want some additional training tips, one site suggests:
This badminton skill requires a lot of practice. It's not easy to do a badminton smash in the middle of the air. It's very common to be inaccurate in hitting the shuttle in mid air as well as losing your balance as you land on the ground at the beginning.
Keep practising until you have the accuracy and body balance.Also make sure that you get the correct sequence. Aim - jump - extend your non-racket arm and lift your legs up at the same time - then swing!
At the start, don't jump too high to enable better body balance and focus on the correct technique.
If you want more power for your jump smash, read this page on generating maximum power for a jump smash.
Gradually increase your jump height over time as you get better at this technique. But make sure you have good balance.
You should possess strong badminton basics to perform this advanced shot.
If you're somehow not able to execute this technique well, review your badminton basics (whether you're holding the racquet wrongly, whether you're doing a full arm swing... etc).
There is also a mention of Peter Rasmussen's badminton coaching video, if you don't mind paying to learn from a championship level player.
This might sound very unhelpful, but really the only solution is lots of practice.
Ideally, get a coach, who can help you improve more quickly, but either way, just be prepared to put in the hours. It will come.
Get a good partner to send you shuttles at consistent heights and positions, and plan for specific targets (eg back left corner, or opponent's head :-) and keep on smashing them. Watch where you make the errors (this is easiest with an observer, or ideally a coach) but you can do it yourself.