When you provide your discount code to customers, they can enter it in a Promo Code field at the bottom of the cart display.
To apply it, they press Apply. The section then shows the discount they received with an option to remove the discount.
They can also do the same from the checkout page. The option appears in the Order Summary section of the checkout process.
To expose a Promo code field, they simply press Add a promo code.
If they apply a coupon code here or on the cart summary, they'll have the option to remove it here by pressing X next to the discount.
The rest of the checkout process proceeds normally.
At this time, customers can apply a maximum of one coupon code per order.