You probably want to divide the items into a few sections such as fruits & vegetables, meat, breads & cereal, etc. listing the appropriate products in the section where you can easily find it if you need to. Leave a few lines after each product so that you can cross out the price and add a lower one if you find a better deal in the future.
Once you have the products listed, (you don't have to list everything all in one sitting — just add more to the sections at the time you purchase something that wasn't in your price book before) the next step is to write down the best price for each item you have ever found. You can use your past grocery lists or the current grocery weekly advertisement flier for the first prices you enter, or just wait until you finish your next shopping run and then add in those prices.
Many people imagine that searching your price book every week for each item will take a huge amount of time. What actually happens is that you'll rarely have to reference your price book. The act of writing down the prices will make them stick in your mind much more clearly so you will only need to reference it on occasion.
With your price book in hand, your goal is to purchase products for less than you paid for them in the past. If the price is less, purchase the product, cross off the old “best price” and add your new “best price.” If the items is a bit more money than your best price, you can make a judgement call on whether it's worth buying and you don't do anything to your price book. If it's a lot more, pass on that product that week.
Another advantage to the price book for many is that it turns shopping into a kind of challenge. You have a goal (to buy the products you have always purchased for less than you have in the past) that you can use to rate how you did each week. Chances are that you won't win the game every week, but you will no longer have to rely on the word of the store that a sale items is really a bargain.