I'm just very frank about it with my 4 yr. old. I prepped him in advance (the Dr. Sears book does a v. nice, simple job with this), and he's asked a few questions about milk/breasts. But at this point, he's seen it all, even when I've pumped - I figured no need to hide at home. Actually, I think the visual of me pumping really stuck with him, he was like, "oh, there IS milk in there" - and then he's been SOOO excited to help dad give baby bottle, so I encourage you start a bottle routine after the first month.
But 5 is great - its not like she's going to run off and get into trouble when you are otherwise occupied like a 2 year old might. W/my son, I've just said, "its time to feed the baby, do you want to hang out in my room or yours?" or "What would you like to do while I feed her??" and we do books, or simple card games. He likes to play games on my iPhone, or cuddle up, or I can just watch him play whatever when we're in his room. I just try not to let the TV be the default all day. The only thing that has taken a bit of adjusting is when he asks me for something mid-feeding. He's def. had to learn to wait 10-15 minutes longer for stuff, which is new, but now that he's not an only child any more, necessary going forward. Indeed, his self-care and self-restraint has increased because of this!
The only thing that was sad was like a week or 2 in he said, "when is daddy going to take that baby and just you and me do something!?" - so again, whenever you can start a bottle routine and get out with your daughter for a bit just the 2 of you, that's v. important. You may just have to explain that its going to take a few more weeks to get there!
Best wishes!