When you talk with your boss, be careful with your words.
Make sure to word things carefully when you talk with your boss.
ご質問ありがとうございます。
① "When you talk with your boss,"＝「上司と話すときは」
"be careful with your words."＝「言葉に気を付けてね。」
② "Make sure to word things carefully when you talk with your boss."＝「上司と話すとき、言葉選びには気を付けるようにね。」
ご参考までに。
You should be careful of the words you use when you speak with your boss.
You should be careful about the words you choose when you talk to your boss.
careful in という言い方はしませんが、その他はどれも使えます。
ーYou should be careful of the words you use when you speak with your boss.
「上司と話す時、使う言葉に気をつけた方がいいよ」
ーYou should be careful about the words you choose when you talk to your boss.
「上司と話す時、選ぶ言葉に気をつけた方がいいよ」
be careful with の場合は、your choice of words のように with の後ろは名詞を使います。
ご参考まで！
アメリカ合衆国 
カナダ 
