On a quote in Store › Quotes, Make an invoice makes a draft invoice in Invoicing with the quote's items at the quoted prices, addressed to the customer's contact. A contact is added when the customer has none. Add shipping or tax on the invoice, then issue and send it from Invoicing.
The quote is marked Answered and links to the invoice, and the invoice links back to the quote. Making an invoice does not record an order or hold stock; use Record an order for that.