How do you get the best interest rate?

No one can promise you a rate, but you can influence the offer you get. Your credit, your down payment, your loan type, and your debts all matter, and rates differ between lenders. Compare official Loan Estimates from at least two lenders, which is always good practice. The loan officers I work with watch rates daily and help you lock a rate given economic changes, and you're free to use any lender you choose. Ask what a lower rate would cost in points and how long it takes to earn that back.