You can use any slightly stale bread that you've got, though usually the simpler the bread, the better the breadcrumbs. To make it stale on purpose, leave it out on the counter for a few hours, or bake it in a 200°F oven for 20 minutes. Tear the bread into pieces and add it to a food processor fitted with the blade. Pulse until the bread turns into crumbs. That's it!