1. Open the Vendors list under Expenses. Click New vendor in the top right. (+ New > Vendor under the Vendors column gets you the same form.)
2. Enter their display name and company name. The display name is what shows up everywhere, dropdowns, bills, reports, so make it something you'll recognize months from now, not just this week.
3. Add their contact details and address. Email and address matter more here than they do for a customer record; you'll want them on file if you ever mail a check or need to send a 1099.
4. Enter their tax ID and check Track payments for 1099, if it applies. This is the step people skip and regret in January. If this vendor is a contractor or business you'll likely pay over the 1099 threshold within the year, flip this on now instead of hunting down their tax ID months later.
5. Set default payment terms, if you know them. Net 30, due on receipt, whatever's standard for this vendor. It pre-fills on every bill you enter for them from here on.
6. Save. They're immediately available in the vendor dropdown on bills, expenses, and checks.