1. Explain it simply
When we already know an action happened, 是……的 emphasizes who did it: [Object] 是 [Who] Verb 的.
这本书是我写的 means 'It was I who wrote this book'.
For a question use 谁 (电话是谁打的？); to negate, put 不 before 是 (电话不是我打的).
2. Show examples
这本书是我写的。
Zhè běn shū shì wǒ xiě de.
It was I who wrote this book.
是 + who + verb + 的
电话是谁打的？
Diànhuà shì shéi dǎ de?
Who made the call?
question with 谁
电话不是我打的。
Diànhuà bù shì wǒ dǎ de.
Sandhi:Diànhuà bú shì wǒ dǎ de.
It was not I who made the phone call.
不 before 是
3. Share a tip
Keep 是 before the doer and 的 at the end. This is different from 红色的 ('the red one') - 是……的 stresses who did a past action.