Breast to formula

Hey y’all, first time momma here. My baby just turned 2 months and has been strictly fed breast milk. I go back to work next month and unfortunately have not built up my supply to feed her breast milk exclusively while I work. I work 12+ hour shifts (nursing). I want to start introducing formula now so her tummy is not in complete shock when I am not with her. Any tips on how to ease my little one into it? I’ve heard mixed things on doing half breast milk and half formula. I also don’t plan to stop breast feeding completely, and i still plan on trying to pump at work. SOS!!