Just leave slices of fresh (not old and stale !) bread on a tray or plate to dry out for a few hours or place in a cool oven until dry - 150 deg C, 300 deg F, Gas Mark 3, (about 15 mins, turn over halfway through) and then process the slices in a food processor or pop them into a sturdy plastic bag and beat them up with a rolling pin. Don't use stale bread - your breadcrumbs will taste stale ! As puddicat has said, you can then freeze them in suitable sized bags or even use a freezer proof plastic container. They defrost quite quickly if you don't pack them into the bag/container too tightly.