Leave slices of white bread on a tray without any cover on kitchen bench for a few day. *2
When the bread dried out, make crumbs using a grater or pulse in a food processor until it makes coarse breadcrumbs.
Notes
*1 I used white bread from a local supermarket. I did not cut the crust off because I don't like food waste. If you want really flaky Japanese panko, you can cut off crust. *2 if you are going to cut off the crust, do it at this point. Also if you are in a hurry and make it quickly, you can put those bread slices into a microwave for one to two minutes to remove some moisture.