Go to Items in the main navigation menu.
Press Custom Attributes in the upper right corner.
Press + New Product Attribute.
Under ATTRIBUTE CODE, enter percentage_discount.
Under ATTRIBUTE NAME, enter Discount %.
Under CUSTOM ATTRIBUTE TYPE, select Product attribute / number.
Under FRONTEND INPUT TYPE, select Text.
Under UI COLUMN SIZE IN POS, enter 2.
You can leave the dropdown menu under SHOW ON QUICK UPDATE FORM blank.
Apply Discount to Product - eCommerce Only
In order for a discount to display on your eCommerce site, you'll first need to create a discount attribute. Once you've created the discount attribute, you'll need to create Discount Start Date and Discount End Date attributes.