Mixing breastmilk and formula!

My baby girl is super petite and even though she’s on her curve, her ped wants me to add 2-3 oz of formula at night time just for the added calories to bump her weight up a little.

How do I mix 2 oz of formula to breast milk? I know I mix separately but then do I mix together and heat together? Or heat separately?

Never used formula before so jw! Thanks y’all 😊