The Cross-sell products option provides an opportunity to sell additional products usually bought with the selected product. Cross-sells are displayed at the bottom of the checkout page.
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Go to Catalog > Products > Manage Products. 
 
- Click Edit for the desired product.
- Click the Cross-sells tab.
 
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Click Add new cross-sell product. 
 
- Check the products you wish to add as cross-sells and select Save. These items will be displayed at the bottom of the checkout page.
 
