In Backoffice, click Lists > Menu > Items to open the Menu Items screen.
On the Menu Items screen, search for your item by clicking the Filter button, and searching for the item number under Item Desc:
Click the Other tab, then enter the commission figure under Item Commission:
NOTE: The figure is in currency value by default. Click Percentage to convert it into a percentage of the item's value.
Repeat the above for any other items that need to have a commission applied.