Cut your bread into chunks and set it somewhere to dry out. It should take 1 - 2 days.  You can slice it, but mine was pretty much rock-hard, so I tore it like this:

Once they're dry, add some to the blender or food processor, filling no more than half full:

Turn it on low at first, slowly turning it up as the pieces turn into smaller crumbs:

Turn it on high briefly to ensure everything's ground up.