If you have any leftover stale sliced bread you no longer want to use, take these remaining slices out of their plastic bag, put them back inside your bread bin and let them dry out by themselves. If you left bread in the plastic bag, mould would soon grow on it but I am sure this has happened to you already!
Then, whenever your recipe asks for breadcrumbs, crush the required amount using a food processor or even a mortar and pestle, etc.
To save time and energy, process a large quantity of dried/hard bread all in one go, cutting the bread into smaller bits to help your food processor, and store in a clean jar for easy use. It should last for weeks, just like the shop-bought breadcrumbs.
Of course, you can also make breadcrumbs using French bread.