formula and breast milk

L
My son is 4 days old. I gave him the colostrum when he was born during skin to skin. I tried breast feeding but it was kind of hard, and I wanted to pump anyways. Well I wasn't getting anything since it had only been a few days. But he was hungry so we started giving him formula. So that's all he's been having, but today my milk has came in. I just pumped and had an ounce but I spilled a little😔 so there's a little less than an ounce(this is the first milk) I put it in the fridge anyways. My question is how do I start giving him breast milk AND still give him formula? I'm strictly bottle feeding him to.