Dear, (имя человека, которому ты отправишь)
In my letter I write about my best friend. Her/his name is (имя подруги или друга). She/he is .... years old. She/he is (tall/short) and she/he has got (big/small)(brown/blue/green) eyes, (long/short)(dark/fair)(straight/curly)(цвет волос) hair. She/he wears (одежда).
(Имя) likes (спорт или хобби) and (животное, написать во множественном числе).
Sorry, I have to go now. Write back soon!
With love,
(Твое имя)