Go to Clients → Add client. Save:
- Display name, company, email, phone
- Billing address (separate from shipping address)
- Tax ID / EIN / VAT (printed on invoices when present)
- Default payment terms (Net 7, Net 15, Net 30, Due on receipt)
- Default currency (Pro)
- Notes (private to you - never printed)
- Multiple contacts (Pro) - primary, billing, shipping
Whenever you start a new document, pick a saved client and every field fills itself.