Create a Promotion to get the same item

Dear All,

I want to create a Promotion for item Category  X1  and the the system apply discount for the selected item on the sales order.

Define Promotion --> Product Attribute = Item Category

Producte Attribute Value = X1

Item1 Category = X1

Item2 Category = X1

Item3 Category = X1

I want to select in the sales order Item1 so the system apply the Promotion for the same item1 and if i select item2 the system apply the Promotion for the item2.

can you inform me how to create this Modifier.

Thanks in advance.