Before starting, make sure your device is completely powered off and unplugged. Use the proper tools if needed (usually included in the package). Do not force anything—if you're unsure, seek help from a professional.
This tutorial will guide you in detail how to remove the Apple A1281 / MacBook Pro 15" Unibody A1286 EMC 2255 (Late 2008-Early 2009) battery.
Step 1
Depress the grooved side of the access door release latch enough to grab the free end. Lift the release latch until it is vertical.
Step 2
The access door should now be raised enough to lift it up and out of the Unibody.
Step 3
Grab the translucent plastic tab and pull the battery up and out of the Unibody.