We are not able to write assignments for you on this site. What I can do is to give you some ideas about things he might want to say. You can make it into a letter yourself by putting it into your own words.
I think that the Giver would want Rosemary to know how much he loved her and how sad he was when she died.
I think he would want her to know what he is doing with Jonas. I think he would want to tell her that this means that she did not die in vain.
She stood up and allowed herself to be killed because she didn't like the society. In honor of that, you could say, the Giver is helping Jonas to completely destroy the community -- to do something that will change it in just the way Rosemary would have wanted.
So I think you should try to write a letter where the Giver shows his sorrow at her death and tries to tell her how that death has caused him to do something that she would apporove of.
We are unable to write the letter for you, but we are always happy to help you out. I would use some facts and emotions to include in the letter. What we know about Rosemary is:
She was the giver's daughter. He had shared loneliness with her. The emotional withdrawal and the lack of ability for her to be able to relate to others in the community following her absorption of the memory led to her request to be released.
The giver is sad at the loss of his child. He feels guilty that he allowed her to be the receiver of the memories.
Think of how you would feel if you were a father to Rosemary. Initially he idea of having your child share your ideas and memories would be nice. You could give her laughter and joy, love and happiness, sleigh rides, Christmas, apple pie and rainbows. You would want to be able to have her to really know what you know.
Then the time would come for you to have to give her those things that had caused you emotional and physical pain for all of your life. She has never felt any type of pain. How could you bear to give pain to your own child? Pain of any kind would be terrible for a father to have to give his own child.
As a father, you think that maybe by giving her emotional pain before physical pain it won't be so hard for her. Instead, she is devastated and wants to have her life cut short. She asks to be released. It is really suicide and you, her father had caused it.
What should you say to your child?