Transitioning from Breast Milk to Formula: Best Approach for Feeding

I am about to return to work soon & will stop BF. I have a stash of breast milk in the feeezer that will probably only last for half a month at most. Should I alternatle between formula and breastmilk (1 formula, 1 breastmilk, 1 formula, 1 breastmilk and so on throughout the day) or should I use up all my breast milk before fully switching to formula? Any mummies in my situation and what did u do?