My sister has downsyndrome, how can I talk to her about periods?

have a sister with downsyndrome who’s 10 years old. She’s starting to develop and the subject of periods has come up. There’s not much advice online about this so my question is what would the best route to go down be trying to deal with periods while she’s in school. We have tried to have the talk but she doesn’t understand fully and she will be scared and not be able to manage it on her own. How do we deal with periods in a special needs child who doesn’t understand? x