FDA guidance for using powdered infant formula is to boil water, let it cool for 5 minutes, then pour the amount stated on the label into the bottle. Add the powdered formula, swirl to mix, and allow the formula to cool to room temperature before feeding your infant. Always use water from a safe source to mix your infant formula. If you are unsure whether your tap water is safe for preparing infant formula, contact your local health department.
Learn more here: https://www.fda.gov/food/buy-store-serve-safe-food/handling-infant-formula-safely-what-you-need-know