The phrase "to mix the formula" means to combine or blend the different ingredients or elements in a specific order or proportion to create a specific mixture or solution. Full definition
I'd say if you can get her interested in eating and drinking by mixing her formula with for example a little bit of fruit, then definitely go ahead. (easybabylife.com)
Can you imagine mixing formula for your child with water that isn't safe for human consumption? (sortacrunchy.net)
There is potential for making mistakes when mixing formula. (cindyandjana.com)