You have to be careful to wash cutting boards really well.
You have to be especially careful when you clean a cutting board.
この場合、次のように言うと自然で良いかと思います。
ーYou have to be careful to wash cutting boards really well.
「まな板を気をつけてよく洗わないとね」
cutting board で「まな板」
to wash really well で「よく洗う」
ーYou have to be especially careful when you clean a cutting board.
「まな板を洗う時は特に気をつけないとね」
to clean で「きれいにする・洗う」
ご参考まで！
カナダ 
