breast milk to formula at 3 months old

Lexi
I'm so sad about transitioning my baby to formula but I'm not making enough to satisfy her. She can empty my breasts completely at every feeding and she still cries when I take her off because my breast is empty. I had to give her formula for the first 2 weeks of her life because I wasn't making enough and she was constantly throwing up for hours after each bottles 
Here are my questions. 
How do you switch an EBF baby to formula? 
Is it normal for her to throw up for hours after? 
Should I mix the formula with breast milk and do half and half for a couple days? 
HELP! I feel like she's starving!