To enable discounts, go to Settings > General > Allow cart-level discount.
Here's how to give a discount on the cart level (over all the products):
Add the products to the cart
Tap Discount above the Pay button.
You'll see 3 discount options:
Add a new price for the product
Add a discount percentage for the product
Add the discount amount
Once you've added your discount, you can apply it and proceed with the payment.