This is an art more than a science but I've found the following helpful:
I find it very difficult to place the lens by looking at the screen - I'm guessing Michael Jordan would not have a problem... But the above advice should work for mere mortals.
- Make sure any bright light is behind you and shining into the HiLO Lens
- Hold HiLO Lens close to the camera but not touching
- You should be able to see the iPhone/iPad camera pupil in the lens
- Center the pupil in the HiLO lens
- Push the lens onto your device
- You are ready!