
Make sure the mounting brakets and ceiling can support what you are storing, otherwise you'll need some bodywork done on your car.
Original article with detailed instructions is here, but just in case I've snatched the steps:
1. Measure:

2. Screw

3. Glue

4. Measure

5. Screw

6. Mark

7.Test

8. Screw some more

9. Put a stopper in

Done!
