"저는 친구에게 편지를 보냅니다."
Translation:I send a letter to my friend.
It would sound a bit awkward to have 저 twice in one sentence, because it's already established that the speaker is the one sending it so it's assumed that it's the speaker's friend. If it was someone else's friend then you would specify with (other person's name)의 친구. Hope this helped lol