I just wrote a detailed post, but the site claimed I wasn't logged in. Grrrrr
I'll Try again..
Ok, I'm not a coach, so first of all, you need to get coaching if possible.
Right, this is if your're right handed - try to take it on your forehand, but if you're caught, try the following. Make sure you have the correct grip as described above.
You need to be in a position with your back to the net, with your right foot parallel to the net. You need to be behind the shuttle, so that it would drop about 40 cm diagonally in front of your foot.
Draw your arm back so that your elbow's slightly above eye level, and your elbow's bent as if you're looking at the time on an imaginary watch on your wrist.
Swing and straighten your arm (keeping the elbow slightly bent), keeping it pronated (shaft pointing straight down) Then suppinate your arm (rotate it 180degrees clockwise.
Make sure you're behind and slightly to the side of the shuttle, your foot's firmly on the floor, parallel to the net, and that the action of swinging your arm is separate from the suppination.
My overhead backhand's definitely my weakest shot, but it's much better now that I've been working on it with my coach. I'm sure there are other techniques, but using this, I can clear baseline to baseline without too much effort, and I don't have to desperately take evasive action when I'm caught by a deep clear.
I hope this is clear enough, and that it helps.