I agree you need to understand the character of Boo Radley. He is a gentle spirit who has been isolated and villified. While Dill has never experienced being villfied, he is somewhat isolated. He spends his summers away from home, and he has to somehow fit in. Dill is not "family" to Jem and Scout, but he certainly feels like it. That is exactly how Boo feels about the three children.
For an assignment such as yours, you always want to try to stay "in character" as much as possible. That is, when you write the letter from Boo Radley of Harper Lee's To Kill a Mockingbird, you will want to consider what his history has been and what type of person he is. For instance, Boo has been a recluse for many years now and has not talked with anyone outside his family. Therefore, his sentences will reflect that he is out of practice. Of course, there is the question of how far he has gone in school, so his writing should probably not demonstrate any kind of sophistication.
As for the content of his letter, if it is written to Dill after the murder of Bob Ewell, there is much that you can include. Since Boo has obviously grown to have much affection for the children, it seems reasonable that the opening to his letter would reflect how glad he is that Dill is all right as well as his sorrow that Dill's arm was so badly injured. (You may also wish to include a remark by Dill that he watched over him as he lay in bed and touched him.) Then, Boo may express his thanks to the children for exchanging gifts in the knothole of the tree, as well as his regret that his brother plugged it up so that he could no longer make things for Scout and him. He may even demonstrate a sense of humor by commenting on how funny Dill looked coming out of his pants that caught on the wire as he struggled to run from the shotgun fire. Perhaps, too, you could include some shy remark of Boo's regarding the fact that he knows that people think he is odd and he realizes that the children were afraid of him. But, now, he hopes that they will think of him in a better way--as a friend. Finally, Boo may ask Dill not to contact him because his brother will be upset. For, it seems from the way the novel has ended that Dill returns to his reclusive life after the death of Bob Ewell.
Whether you want to include mention by Boo about Bob Ewell is one that depends upon your interpretation. Somehow, it seems that this shy man may not wish to discuss what happened. Certainly, he said nothing to Scout or anyone at the Finches' home.