1. The parents should be your partner in tackling this issue as it is mostly going to be hands on teaching of social manners and graces. Even if an only child, they need to make her wait more often when asking for things instead of immediately giving them to her, practice conversation skills especially not interrupting and taking turns talking at meal times, when traveling in the car, at home, and just about anywhere.
2. Look up Montessori manners and social grace lessons and practice one or two at a time at circle times, lunch time and whatever fits.
3. Point out when someone gets its right but do not over do this where it is unnatural. I like how Mary waited for her turn, please tell me Mary.
4. Have a private lesson on this manners lesson with the child. "I love how you are full of knowledge and ideas and are eager to share them but there is not enough time in the day to tell me everything you think of so when it is your turn at lunch next time, please think of one thing to tell me. This way everyone gets a turn. Then, if we have extra time, you may have another turn after your classmates. At this same time discuss emotions and polite ways to express them.
5. Set up a calming spot in the room for anyone who needs to use it. If the child cannot help herself and keeps interrupting, ask her to go sit in the calming spot for a bit focusing on what she can do to give others a turn (sitting nicely and listening, not talking when not her turn). And, to return when she is ready to be a listener not just the speaker.
6. Teach the others how to address it with her with their words. Teach as a group or small group lesson how to remind others or how to ask others to please let you finish then they may have a turn.
7. Remind, represent lessons, repeat.