Your back is the platform you lift from. Your shoulders blade should be held back and together. Keep them there throughout the lift.
The only movement should be in your arms. The rest of your body should be completely still, stabilising you.
Bring the bar down about nipple height, let it just touch the skin, pause and then reverse the movement. Don't jerk the weight, smoothly, squeeze the bar up. Lower it down slowly and under control.
If you lift like this you won't have any problems.