Breastfeeding help

My daughter had her one month check up today, and although she has gained weight she hasn’t gained as much as they would like. So, the doctor wants me to start feeding her for 15 minutes on each side and then follow it with a bottle. Before I was feeding on one side only for 5-35 minutes (she decided) and had a haakaa on the other side which I would use to make up some bottles for her.

My question is, with feeding her on both sides each time how do I make her bottles?

Thanks!