To make a discount on a specific product, go to "Products" and choose the product you want to discount. Press on the edit button. 
 

Scroll down to "Product discount" and enter either the discount in percentage or in DKK. When entering one the other one will appear automatically. 

When you have entered the amount of your choice scroll down and press "Save".
 

Now your product will appear with the entered discount, and your costumers will know this product is on sale.