First click the category you wish to add the item into and then from left
navigation menu click "Create a new item"
Enter information and click "Next".
You can enter rules for the item. Check FAQ: Item Rules
for more information. You can add additional information for the item by clicking
"+add" near "Additional Item attributes".
You have 4 types of item attributes here(Professional and Developer eShop).
Enter a name, type and value. Check FAQ: What are Item attribute / variants? for more information.
Click "Finish". Click "Finish" again to complete the item set up.