How could a reader write a letter to Daisy as if they were Gatsby? The letter should be the one that Daisy reads just before her wedding and...

Wow, what a great assignment! 

Well, some important things to consider here would be voice, conflict, and themes. 

Voice: The tone, diction, and syntax that create a characterIn The Great Gatsby, there is a lot of evidence you can use to write the way that Gatsby would have written. Look at the kinds of words and phrases that he uses regularly (Old Sport), his sentence types (are they long? short? questions? demands?), and...

Wow, what a great assignment! 

Well, some important things to consider here would be voice, conflict, and themes. 

Voice: The tone, diction, and syntax that create a character
In The Great Gatsby, there is a lot of evidence you can use to write the way that Gatsby would have written. Look at the kinds of words and phrases that he uses regularly (Old Sport), his sentence types (are they long? short? questions? demands?), and even the evidence of his writing that we receive in the final chapter. This will make your letter read really authentically. 

Conflict: What disagreement or incompatibility exists between the two characters and their worlds?
Consider what sort of things might drive Daisy and Gatsby apart. Gatsby's letter should address those, and then prove that they don't matter or that, together, they can overcome them! Daisy is from old money, but Gatsby is poor; this should be a main point of the letter. 

Themes: The main ideas or concepts of the text
The Great Gatsby is a seminal work on the American Dream. A good point to consider is that having Daisy is Gatsby's American Dream; addressing this point might drive her to tears!

Good luck!