Let's go through some things to look out for as you add to your list.
Stores: I compared prices at Meijer, Family Fare, Fresh Thyme, Sam's Club, Costco, and Aldi, since those stores are local to me. Aldi is a bit of a hike, as is Fresh Thyme, and Sam's Club and Costco are membership based (but each of our parents has access to one of those membership, so yay for us. If you're on a tight budget, you may want to find a friend with a membership or split one with someone). I also want to find a few local farmer's markets when the weather gets warmer, so I can compare prices there. Even if you think Whole Foods, Fresh Thyme, an organic market, or a farmer's market are out of your budget, you might want to go through and fill out your list there, since it may surprise you (for example, produce at Fresh Thyme is cheaper even than Meijer, I've found, and they often run specials).
Name: I am using this as the generic type (i.e. apples, rice, laundry detergent, beef, etc.).
Brand: This is the specific type. Fresh v. frozen, the fresh is the default for me, so I don't bother adding fresh behind produce, however I'll add it to mozarella because the default there is shredded/bagged (figure out what your defaults are and enter data accordingly). I buy most produce fresh, but I do buy frozen corn, so that would be Name: Corn, Brand: Frozen. Then also the sub-type, so Fuji, Pink Lady, Honeycrisp for Apples. Or the actual brand, i.e. the frozen juice we buy would be "Frozen, Old Orchard" or "Frozen, Minute Maid."
Quantity: the size it comes in, or the amount the price applies to. Some common ones - oz. (ounce), lb. (pound), ea. (each), 2L (2 Liter), can, 8 oz., etc. When it comes to cans and packages, I prefer to write down the amount in the can or package, because can and package sizes can vary. Often the signs will also tell you in very small print what the price per each or price per ounce is, so then I'll just do oz. and write down the per ounce price under the applicable store. Where quantities differ from store to store, this is especially important.
Under each store you chose: put down the dollar amount. If you are looking at by the pound ground beef, make sure you are comparing similar types of ground beef (85/15 versus 70/30 are going to be different), check the quantity on all packages so you're not comparing a 18 oz. bag at one store with a 22 oz. bag at another, also, check the value packs, those generally have the lowest prices. I write down whatever is lowest, knowing that if I do decide to buy something it will be the value pack and then I'll just break it up into smaller bags and throw the extra in the freezer for another meal.
Here is a portion of my list. I haven't filled out everything for every store. And as you can see, I am comparing pounds of lemons at Meijer and Family Fare to by the each at Fresh Thyme, so that's a little problematic.
To simplify your list: I like to go through and divide out differing amounts, so if Family Fare has 4 peppers for $5, I try to get it down to by the each. I also like to divide out by the size of the packs, so that I'm comparing ounces to ounces instead of a 18 oz. pack to a 32 oz. pack. If I look at two different sizes of the same cat food, and one is clearly cheaper, I remove the more expensive one from the list. Same with name brands, unless we greatly prefer one over the other, I'll remove the more expensive one. Don't forget to compare you favorite brand of toothpaste, deodorant, etc. although you may want to add some drug stores or a pet supply store into the mix for better comparison purposes.
How to use the list:
Knowing what to buy where. I'm not saying you need to go to every grocery store every week. That would negate your savings in gas and wear and tear on your car. I alternate going to Costco and Sam's Club once a month. After comparing prices between Family Fare and Meijer, I'll probably drop Family Fare from my list completely, because Meijer is cheaper across the board for my weekly shopping trip. And Aldi is only worth a trip if I know that several of the things I need that week are significantly cheaper than at Meijer, or if I want to stock up on fruit snacks, cereal bars, or pop tarts and other things that won't go bad very quickly.
Having a baseline idea of what things cost is helpful when you see a deal. Even if I go into Meijer for five things, I usually buzz by the meat section to see if they have any really good deals. Or you can read the online sales leaflets (or the paper ones, if you get those in the mail). We get most of our meat from Costco or Sam's Club, because it's just cheaper there and our chest freezer is rather large. However, sometimes the Meijer deals will blow the base prices at the other stores out of the water, and then I'll buy one or two to throw in the freezer. Spend a little money now to save a lot later on. Just make sure you're not buying something only because it's on sale. That's a waste of money if you're never going to eat/use it.
It's also good for comparing large amounts of things you know you'll use up, versus the smaller containers or by the each items. So if I know that Fuji apples are $1.99 per pound at Meijer for loose apples, and Sam's Club has a 3 pound bag of Fuji apples for $4, I know that it's cheaper to buy the bag (IF we are going to eat them all - i.e. I'm going to be making apple sauce or something). If you buy the bag and only eat 1 pound of apples before the rest goes bad, you've wasted money.
It lets you cost out your meals before you head to the grocery store. I meal plan and shop once a week (unless we run out of something important like milk halfway through the week). Knowing what my weekly budget is for groceries, plus what I already have on hand from shopping deals and Costco/Sam's Club, helps me when I sit down to make my meal plan. If we're really tight on money, I try to stick with the stuff we already have, plus what's cheap to buy at the store. I also try to keep a bit of wiggle room in that budget, in case I do see a really good deal while I'm out.
Buying name brand instead of generic. Sometimes I'll find a coupon or a deal so the usually more expensive name brand is cheaper or the same price as the generic. This matters mostly for things where you actually care about the taste, but maybe name brand just isn't in the budget. For me that's ice cream. I can always get generic for $2.99 a quart, but sometimes Hudsonville ice cream or Breyer's is marked down and then I like to get those instead, because I just like the taste better, but generic ice cream is better than no ice cream in my book. Also, ice cream is great freezer fodder if you have a large chest freezer like we do. Because it runs much more efficiently when it's full, we try to keep a bag of ice in there, and cheap stuff like ice cream and bread, so we're not filling it to the brim with expensive meats. The opposite may also be true. I don't like the generic Sam's Club diapers, so I buy Pampers when I go there. However, the Kirkland brand at Costco is just fine. So I like to be able to compare the lowest cost Pampers at Sam's Club with the generic (Kirkland) brand at Costco by the each by size (my kids are in Size 2 and 5 right now).
Shopping elsewhere. Maybe you stumbled across a new grocery store or a farmer's market/stand on a route you don't usually take. You've got your comparison list and your grocery list in your pocket or on your phone or in your planner. And because you're already here, it's not wasted gas because you weren't going to multiple stores.
