1. Explain it simply
因为 introduces the reason ('because…'), 所以 introduces the result ('so…').
Unlike English, Chinese is comfortable using both words together in the same sentence.
2. Show examples
因为下雨，所以我没去公园。
Yīnwèi xià yǔ, suǒyǐ wǒ méi qù gōngyuán.
Because it rained, I didn't go to the park.
因为 + reason, 所以 + result
因为他很努力，所以考得很好。
Yīnwèi tā hěn nǔlì, suǒyǐ kǎo de hěn hǎo.
Because he worked hard, he did well on the test.
Cause first, then effect
他病了，所以没来上课。
Tā bìng le, suǒyǐ méi lái shàngkè.
He was sick, so he didn't come to class.
所以 alone is fine when the reason is already obvious
3. Share a tip
You don't always need both halves - Chinese speakers often drop 因为 and just use 所以 when the reason was already stated or is obvious from context.