How to start formula?

Kate

I’ve EBF for almost 9 months. This past weekend I got mastitis again and my supply isn’t coming back. Instead of pumping 6 ounces, I am consistently pumped 2 or 3 total. It’s frustrating. I think I want to start supplementing with formula but don’t even know where to start. Can I both breastfeed and give formula to my baby? How do I introduce? FTM and no idea!