Your question is too important and complex for a forum such as this. You need to have someone competent show you their technique. You can read about techniques in Up-to-date, but in real life, it's often different. And you are only as good as the person who trained you.
The way I learned it:
1. You must establish excellent rapport with the patient. This is the most important step in any procedure (especially one that causes pain).
2. You must adequately anesthetize the site. Using the lidocaine needle, you must "map-out" the topography of the iliac crest using feel alone until you have a mental picture of the site.
3. You double check the position of the Jamshidi using your mental image and feel. This must be positioned right so that it doesn't slip with pressure.
Please get someone to teach you in person. If the patient is too obese, I am beyond my comfort level. It may be time for interventional radiology.
Good luck.