Introducing formula

Currently I breastfeed my son. I pump at work and after he nurses. Unfortunately, since going back to work, my supply hasn’t been able to keep up with my baby’s needs. I’ve been using all of my frozen milk to give him a bottle before bed. I’ll nurse him and he’s not satisfied, so I’ll give him a bottle. Once my frozen stash is gone, I’ll have to move to formula. I’m a FTM and have no idea how to introduce formula. Is one bottle a day at night going to affect anything? Do I mix my milk in with formula to start or just do straight formula? Any help or insight is greatly appreciated!