When you have Stock Management set up, any wastages on the POS will be recorded in your stock levels. You can also stock out and record wastage directly from the backend.
On the backend:
Go to Stock Management > Stock Level
Click the pencil button
3. Click Stock Out
4. Select the Location and enter the Stock Out amount. If this is wastage, tick Is Wastage and select a Status (e.g. "expired").
5. Click Save.