To create a customs invoice or import-export declaration in Odoo Community Edition, start by installing the Inventory and Invoicing modules and configuring company details. Add product-specific customs information like HS codes, country of origin, and weight. Create and validate a sales or purchase order, then export the invoice as a PDF. For declarations, export data in CSV or Excel and map fields like HS codes and values to match customs templates (e.g., CERS declaration). Optionally, integrate with CERS via API or middleware. Finally, store customs records and shipment proofs for compliance and audits.