To apply a percentage discount for a particular product please go to Products and select the product on which you would like to apply the discount. On the product edit/add page you can find a discount box:
Switch a discount type to “%” and enter the discount value (20 for 20% discount, 30 for 30% etc.). You can apply a discount from certain quantity. If you want to apply a discount for any quantity just leave it with the value “1”.
That’s it! Now save the product and watch the sales!