By aisle or expiration date? Alphabetically by brand or product name? Everyone has their own system of organizing coupons, and the only right way is the one that works for you. To decide whether you ought to file that coupon under "frozen foods," "Eggo," or "waffles," think about how you tend to shop.
Do you shop primarily in one store, or do you go in search of the best prices every week? Are there must-have brands that you purchase week after week, or does the best price win? If the store where you always shop keeps the juice boxes in the cereal aisle, for example, then it might make sense to put those coupons near each other in your book. But if you visit different stores every time you shop, it might be easier to sort by category (cereal, beverages, snacks, etc.) or even product name (Cheerios, Fruitables, Oreos).
Whatever system you go with, keep it simple, and remember, you can always change it later if it's not working for you.