Answer:
Most coupons have a bar code on them that the cashier will scan just like the rest of your groceries. They will tell the computer to automatically deduct the proper amount from your total cost. Other times (like at fast food restaurants), the cashier will read the coupon and manually change the price for you. Online coupons are usually alphanumeric letter string that you type in when you place an order on a website during the checkout process. See the page link, listed under Sources and Related Links, for an article that teaches you how to make the most of coupons.
More information:
When you need to buy something you can first search for coupons online. This produces plenty of sites to choose from. Among those pick 2 or 3 sites which are suitable and use the coupons from those sites. Then bookmark the good sites and whenever you want coupons you can return to those sites. For a list of possible sites, see the page link, further down this page, under Related Questions.