You can quickly apply a check discount to a check as a whole in the ordering screen. Tap the "Discnt" button at the bottom of the check.
On the next screen you will see the discount options displayed. Again, you may have additional options that have been configured by a manager or other administrator.
Next, choose which discount should be applied to the entire check. The discounts that appear have been configured in the Discounts section of Toast's back-end. For more information on configuring custom discounts, please refer to this article.