Сначала пишешь приветствие; затем благодаришь за письмо; пишешь про приглашение; извиняешься и говоришь, что тебе нужно спешить; прощаешься.
Пример:
8.03.2014
Russia, Moscow
Hi, Ann
Thank you for your letter, I'm happy to hear that you are fine. I'm hope that you have a good holidays!
So, in this month I have a weekend and I'm really miss you. My mom and I want to see you in our house with your family. Can you gone to us? I will very happy if I see you here.
Sorry, now I should do my homework, because I have a lot of it.
By for now
/Твое имя/