This could be a really fun and interesting assignment; the important thing, however, is knowing what key events happen in the novel. The three young characters in Harper Lee's To Kill a Mockingbird--Dill, Jem, and Scout--are always getting into trouble, and most of it is somehow connected to the Radleys. To write this letter, you just need to imagine you are watching some of their antics from inside your house and then reacting to them.
For example, when Jem gets mad at Scout and pushes her too hard when she is in the tire, this is what Scout experiences:
The tire bumped on gravel, skeetered across the road, crashed into a barrier and popped me like a cork onto pavement. Dizzy and nauseated, I lay on the cement and shook my head still, pounded my ears to silence, and heard Jem’s voice: “Scout, get away from there, come on!” I raised my head and stared at the Radley Place steps in front of me. I froze.
Think about how that must have looked to anyone inside the house who happened to be watching. Here comes a tire, Scout's overall-covered legs sticking out over the sides. The tire rolls swiftly and bumpily toward you until it hits something and pops a dizzy little girl onto the ground. Scout says she was "dizzy and nauseated," so what do you think she looked like as she was trying to recover her wits? I am amused just visualizing it now, so you should be able to think like Boo and describe what you see--especially as her brother starts yelling at her and she finally realizes where, exactly, she is. Later Scout reveals that she heard someone laughing, and certainly that must have been Boo.
We know that Boo watches the children, especially Jem and Scout, because he carves their likenesses into bars of soap. He had to have seen them walk to and from school every day or he would not have known where to leave them their gifts in the knot of a tree. If I were Boo, I would certainly have been watching after I left something there, anxious to know if either of the children would stop and, if so, what their reaction was to the gifts.
We know that Boo laughs at their antics, and we know he is aware of their actions. He appears during the fire and gives Scout a blanket to keep her warm, and he knows it was the kids who were sneaking under the fence when Nathan Radley appeared with a gun. Perhaps it would be interesting for Boo to explain why he not only returned Jem's torn overalls but also sewed them. He certainly did not have to do that, but something prompted him to do it. Think about what that might have been.
How about watching the kids build their silly snowman, put on dramatic plays, or even play the "Boo Radley game"? Boo had to have guessed that he was the star of many of the children's productions. I don't know how far you have already read so I won't give any more examples, but there are plenty. Just think of at least one silly or ridiculous thing the kids do that Boo must have seen and start writing. If you need more incidents, there are plenty of things from which to choose. Talk to the kids as if you were their friendly neighbor who never gets to talk to them but really enjoys living next to them; that should be simple because that is exactly who Boo Radley is. He is a rather child-like person, so give him the voice (through your writing) that you think he should have. Happy writing!