1. Explain it simply
是 … 的 emphasizes the time, place, or way a past action happened.
Put 是 before the detail you stress and 的 at the very end: 是 + detail + verb + 的.
Both parts are needed - say 我是昨天来的, not 我昨天来.
Use it only for actions that already happened.
2. Show examples
我是昨天来的。
Wǒ shì zuótiān lái de.
I came yesterday.
emphasizes time
他是坐飞机来的。
Tā shì zuò fēijī lái de.
He came by plane.
emphasizes manner
我是走路来的。
Wǒ shì zǒulù lái de.
I came on foot.
emphasizes how
3. Share a tip
是 marks the detail you want to stress and 的 closes the sentence - keep both, and use the pattern only for past actions.