I saw a letter that someone wrote to Santa taped in one of the windows of their house.
Someone wrote a letter to Santa and stuck it on the inside of a window in their house, and people can see it when they walk by.
この場合、次のような言い方ができます。
ーI saw a letter that someone wrote to Santa taped in one of the windows of their house.
「誰かがサンタに書いた手紙が家の窓にテープで貼ってあるのを見た」
to write to Santa で「サンタに手紙を書く」
to tape in one of the windows of their house で「家の窓のひとつにテープで貼る」
ーSomeone wrote a letter to Santa and stuck it on the inside of a window in their house, and people can see it when they walk by.
「誰かがサンタに手紙を書いて、家の窓の内側に貼ってあったので、その家を通りかかるときに手紙が見える」
to stick it on the inside of a window で「窓の内側に貼る」
to walk by で「通りかかる」
ご参考まで！
カナダ 
