To avoid injury, do not lower the bar much further than the top of you head when bringing the bar down behind you. This will keep your arms at a 90 degree angle to the floor and place less stress on your shoulder joints.
Keep your back straight throughout the movement. Dont let it arc too much when pressing the weight.
Use a lighter weight that you would on a regular barbell press.
Use slow and controlled movement, both when pressing and when lowering the weight.
Instructions
Set up for the smith machine shoulder press behind the neck by setting a bench down in the smith machine and adjusting the back to a 90 degree angle.
Now sit down on the bench and adjust the position so that the bar comfortably comes down just behind your head.
Next, add the weight you want to use and sit down on the bench.
Un-rack the weights and bend your elbows slightly. This is the starting position for the movement.
Slowly begin lowering the bar down behind your neck as far as comfortably possible.
Once your upper arms are slightly past parallel to the floor, pause, and then slowly raise the barbell back up without locking the elbows out at the top of the movement.