Having a grocery shopping plan is not the same as having a list. A list helps but there needs to be more if you are going to keep your shopping with-in your budget.
One tool I have been using is my Kitchen Inventory. Quick recap from the post about that - KI is an excel spreadsheet broken into areas in my kitchen and food groups that gives me a fairly accurate account of what I have or do not have stored, along with the last price I spent. I made a mistake this past month in not printing the KI after I had inputted prices. I had taken the document with me as I shopped but I didn't have the prices to compare (and was shocked at how drastically some changed). That won't happen again since I am finally understanding how best to use this.
What the KI does, aside from price comparison, is tells me what we use and gives me ideas how to combine new items with the old to make a meal. For example, staples - tortillas, cheese; ingredients at home - diced chiles, green salsa. If I buy a can of refried beans, we could have bean burritoes or I can buy some meat and make tacos or different burritoes. There's no second guessing about what we have and what we don't have.
I mentioned that tortillas and cheese are staples. Using the KI, I can determine what foods are necessary in my family and what we don't use as fast as I thought and what we don't really use at all. I make notes on my KI so I know when we didn't like something or a brand. That will save me money because I am not buying something we won't use.
For us - staples are pasta, tortillas, cheese, flour, sugar, eggs, peanut butter, Nucoa, rice, yeast and garlic. With those items in our house, we will always have something to eat. They are also what I replace before I replace anything else.
Then I plan meals - I don't do a hard plan like: 1st Roast chicken with mashed potatoes, green beans; 2nd meatloaf, green salad; etc. I take what I know we like and think of additional meals we might like to have. We love pasta so I plan for spaghetti with marinara, pasta carbonara, mac and cheese. Note: I don't buy processed foods. They may feel like a bargain but in the end they are not. I may be able to purchase a box of mac and cheese for 50 cents but it could take as many as 3 boxes to feed my family without taking in account the milk or the butter. For $1 I can buy a pound of elbow macaroni, use powdered milk (free), 1/2 stick Nucoa (.20), flour and 1/3 pd of cheese (.86) making the whole amount 2.06 vs the cost of the box mac and cheese. That might not feel like a bargain compared to the 1.50 but think of this - the homemade mac and cheese will be more filling and more nutritious. There's less sodium. I can easily alter either mac and cheese to include veggies and meat but with the homemade - I can make extra sauce to use for lunches or dinner the next night. That extra .50 can make a big difference. More importantly, all the ingredients for the mac and cheese are included in my staples so I don't have to really think about what to make.
The last Plan tip I'm gonna share today: traditional American meals of meat, vegetable, starch are expensive and lack nutritional variety. Think past the traditional dinner and start thinking one dish meals. Not everything we eat is one dish but when you start thinking in this manner all sorts of things open up. Meat and fresh produce are some of the most expensive items in the grocery store. Shopping in season and sales will help but I find that creative one dish cooking helps more.