I don't have templates or the like because each case is different. Essentially, you need to relate specific condition/symptoms to specific duty limitations for your MOS/AFSC/Rate.
BAD: Service member's conditions prevent performance of his duties.
GOOD: Due to the member's limitation of range of motion due to his back conditions, he cannot climb in and around aircraft to perform maintenance as is required of Aircraft Maintenance Technician. In addition, his back pain limits his ability to stand for more than 25 minutes at a time, thereby making him unable to perform routine maintenance or to work on the flight line. His back pain on weightbearing also prevents his ability to wear Individual Body Armor and Kevlar and prevents his ability to qualify with his assigned weapon.
The above was just a general description/example. The point is that a strong letter addresses conditions/symptoms and their relation to duty performance.