There are two things you should consider as you work on this assignment, the conventions of Renaissance letter writing and Romeo's speeches about the reasons for his suicide.
The letter should begin with a formal salutation, such as "To my most noble and honorable father, the Lord Montague, from his most unfortunate son Romeo." The second part of the letter normally captures the goodwill of the audience and includes such things as wishes for the continuing health of the recipient.
The narration, or main body of the letter, should describe the circumstances of Romeo's suicide, including his receiving the news of Juliet's death from Balthasar, his purchase of the poison from the apothecary, his arrival at the tomb, his fight with Paris, and his final glance at Juliet before swallowing the cordial. After the narration, Romeo should expand upon the reasons for his committing suicide, including his unwillingness to live without Juliet.
The conclusion of the letter normally expresses wishes for the future. This might be a place for Romeo to express a desire for an end to the feud between the families.