"A girl sends a letter to a boy."
Translation:여자아이는 남자아이에게 편지를 보냅니다.
For the 를, the ㄹ is somewhere in between R and L. It's kinda like a combination of both.
If you find it difficult pronouncing 를,
Try saying: "Lil"
but stress the L a bit to make is sound a little bit like R.
If you can't stress the it to make it sound a bit like R, then just say:
Hence, 편지를 is prounounced: Pyeonjilil
...stressing the "l" of course..
I hope this helps...