It all started when Caillou's Mommy was putting on her make-up, getting ready to go out.全ては、カイユのママが出かけるためにお化粧をしている時に始まりました。
■ put on one's make-up ■Phrase[]お化粧をする
■ , doing ■Participial Construction[理由原因(because)]
It all started when Caillou's Mommy was putting on her make-up, getting ready to go out.