Another of the large number of collocations in Japanese based on 気（き） ‘attention’, or ‘focus’, is 気（き）を付（つ）ける. This is just a combination of the noun 気（き）, を, and the る-Verb 付（つ）ける ‘to adhere’, or ‘to attach’. 気（き）を付（つ）ける primarily follows (A) に. This marks (A) as something that the speaker is ‘adhering’ their attention to, and translates as ‘to pay attention to (A)’, ‘to be careful of (A)’, or ‘to watch out for (A)’.
This grammar pattern is used after nouns or noun-like phrases, indicating the thing that is being warned about, or that the speaker themselves is being cautious of.

• 最近（さいきん）、電車（でんしゃ）に痴漢（ちかん）が沢山（たくさん）いるらしいから怪（あや）しい人（ひと）に気（き）をつけて帰（かえ）ってくるんだよ。
  I hear there are a lot of perverts on the trains these days, so watch out for suspicious people on your way back home.
• 車（くるま）で歩道（ほどう）を横切（よこぎ）る時（とき）には歩行者（ほこうしゃ）に気（き）を付（つ）けましょう。
  Be careful of pedestrians when you drive through a sidewalk.

Most often, 気（き）を付（つ）ける will be used to warn about possible errors that could be made, mistakes, or simply the loss of focus. Due to this, it frequently follows negative verbs as ないよう, highlighting that something should be done in a ‘manner’ that doesn’t result in (A).

• このギターは俺（おれ）の姉（ねえ）ちゃんのやつだから壊（こわ）さないように気（き）を付（つ）けてね。
  This guitar belongs to my sister, so please be careful not to break it.
• 今月（こんげつ）はお金（かね）を無駄（むだ）遣（づか）いしないように気（き）を付（つ）けよう。
  This month, I will be careful not to waste money.