Switching from pumped milk to formula

Can someone please tell me the best way to switch my baby from my pumped breast milk to formula? I have been pumping for 12 days since she wouldn’t latch and my supply is down to 3 ounces every 3 hours and she eats almost 3 ounces every 2-3 hours. Sometimes she gets hungry and eats closer together and I want her to get full and have enough! I’ve been stressing with pumping, it’s just not for me. It’s causing me depression and I can’t stand having to pump when my little girl needs my attention . So if you’ve switched from breast milk to formula how did you do it? Did you put half of each into the bottle or do formula one feeding and breast milk the next?