At some point shortly after you have gotten your groceries put away, it would be a good idea to update your inventory. This is not necessary. You can always start back at Step 1 next week, and re-evaluate your inventory using your Master Grocery List. However, if you’re a large family, or have a large stock on hand, it’s never a bad thing to keep track of what you have so that you can make use of it in your menu planning.
If you are not the inventory-keeping-type, my next suggestion is to have a piece of paper or a pad of paper specifically for the purpose of your grocery list ON DISPLAY in your kitchen. The MOMENT you realize you’re running low on something, PUT IT ON THE LIST IMMEDIATELY. If you can keep this list on hand, it will make your menu planning/grocery list process easier next week.