Write an invoice and finalize it

Open Invoices for your client and choose New document. The Document type is Invoice by default.
Pick the customer with the customer search (name, number or city) and add your Lines: what you sold, the quantity and the price. Check Payment terms and Due on.
Choose Save. The invoice stays a draft, so you can still change it or remove it with Delete draft.
Look at the result with Preview (PDF) before you go on.
When everything is right, choose Finalize and then Assign number and lock. A finalized invoice has its number and cannot be changed any more. It is booked as income.
If something is missing, the app lists what to fix and the invoice stays a draft. A mistake after finalizing is fixed by a cancellation or a corrective invoice, not by editing.