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 'Related' tab.

5) Under the 'Related Books' heading, enter your item's related items by their Item ID.  For multiple related items, separate the Item IDs with a comma.

6) Click the 'Add Book(s)' button.

7) When you are finished editing your item's related items, scroll down and click the 'Save Changes' button to finalize.