If you keep her with the flock, you'll have to mark all the eggs under her so you can remove any added later.
Leghorns aren't supposed to go broody, but they do. We used to keep 100 white leghorns for egg sales and there were usually anywhere from 1 to 3 in broody jail.
I had a black leghorn that raised about 5 batches of chicks for me.
Is your rooster a leghorn?
In reality, you don't need to do anything but mark the eggs and remove volunteers.
In 3 weeks, you'll have chicks.
She will come off the nest every day or two to eat, drink, defecate and stretch. You don't have to put food and water by her, she'll find it where it has always been. You can tell she's off the nest because she'll do the broody cluck while she is out and about. If it is hot, she may stay off up to an hour or more. If it is cold, she may only stay off for 15 or 20 minutes.
Often I'll put the broody in her own quarters if I have a unit available.
I can count the huge stinky lumps of feces to know if they come off the nest and I'm here to tell you, they don't always come off the nest every day - but they won't starve themselves to death. Just leave them alone and count chicks in 3 weeks.