A shared shopping list fails when it is easier to mention an item aloud than to put it on the list. It also fails when three people add milk without checking whether someone already did. Keep the system simple enough to use while standing beside an empty shelf.
Choose one place: a shared note, a paper pad or an app the shoppers already use. Avoid maintaining several lists for the same trip. If someone cannot access the chosen format, agree how their request will reach it rather than assuming they will remember to repeat it later.
Record the useful detail
Write the item and the quantity needed. Include a size or specific type only when it matters. Yogurt for lunches may require more detail than apples, especially when a household member has a dietary requirement. Keep essential restrictions explicit rather than expecting the shopper to infer them from a brand name.
Add an item when the remaining amount reaches the point at which you normally replace it. For some things that means the last packet; for others it means a nearly empty container. The household does not need a complicated inventory, but it does need a shared understanding of what running low means.
Before shopping, check the refrigerator, freezer and cupboards. Remove duplicates and reconsider items attached to canceled plans. A list is a working note, not a promise to buy everything that was ever added.
Agree on substitutions
Tell the shopper which choices are flexible and which need confirmation. A different pasta shape may be fine; a product with different ingredients may not be. If an essential item is unavailable, a brief message is better than making an uncertain substitution that cannot be used.
Mark purchases as they happen so another person does not buy them on the way home. If two adults shop separately, split responsibility clearly rather than relying on both to notice a partially completed list in real time.
After unpacking, add any genuinely missing item and close the old trip. Keep household supplies on the same list if that suits the shopping route, or use a clearly separate section. The system should reduce remembering, not create a second administrative task after every purchase. If people stop using it, make the next action easier before adding more features.