Place the cubed bread in your food processor and pulse until fine crumbs form. You now have fresh bread crumbs!
To make dry bread crumbs
Spread the fresh bread crumbs on a baking sheet (lined with parchment paper for easy clean up, if you prefer). Bake at 250 degrees F for 15-20 minutes, stirring around every 5 minutes or so, until bread crumbs are dried out. (Depending on how stale the bread is and what kind, it could take up to 30 minutes or so for them to dry out.)
Recipe Yield: 4 cups of cubed bread will yield about 2 cups of bread crumbs.
Breads to use: Leftover artisan bread, focaccia, leftover hot dog or hamburger buns, dinner rolls, sandwich bread, or almost any other bread you have lying around to make these bread crumbs. Gluten-free bread, whole grain bread, white bread are all fine.
Breads you may not want to use: Sweet breads (such as challah, Hawaiian rolls, etc.) if you don't want the bread crumbs to taste sweet. Breads with seeds - they will add a crunch to your breadcrumbs, so don't use unless the recipe will be OK with a little crunch.
How to store fresh bread crumbs: If you aren't using right away, store fresh bread crumbs in an airtight container or bag in the refrigerator for about a week, or freeze for about 4 months.
How to store dry bread crumbs: Store cooled dry bread crumbs in the pantry in an airtight container for 2-3 weeks (be sure they are truly dried out and completely cooled for them to last!), or freeze for about 4 months.
How to make them without a food processor: A blender is the best substitute. You can also place dry or toasted bread in a zip-top bag and use a rolling pin or kitchen mallet to bang the bread to make crumbs. A box grater will also work (carefully, though!) on dry, stale, or toasted slices of bread.