1) Click on "Manage Inventory" under the 'Inventory' heading of the Control Panel.

2) Search for your item by it's Item ID (or Author / Title)

3) Scroll down to the 'Search Results' and click the 'Edit' link under your item.

4) Click on the 'eBay Properties' button.

5) Scroll down to the 'Listing' heading and uncheck 'Use Auto Description?'.

6) Edit your item's 'Description'

7) When you are finished, be sure to click the 'Save Properties' button to finalize.