Introducing formula to breastfeed baby

Nanay

Hi Mommas! My baby is 7mos old today and is eating bit of solids and still breastfeeding. In January next year I will go back to work, I know I still have lot of time but I want smooth transition when she switch to formula. When did your little cuties started with formula? Any tips for me how to introduce it? P.s My baby is nursing to sleep.