1. Explain it simply
The 被 passive is Receiver + 被 + (doer) + verb + result.
The receiver, the thing affected, comes first, before 被.
The verb still needs a result like 走, 完, or 坏: 被弟弟拿走了。
2. Show examples
我的手机被弟弟拿走了。
Wǒ de shǒujī bèi dìdi názǒu le.
My phone was taken away by my brother.
receiver + 被 + doer + verb
蛋糕被妹妹吃完了。
Dàngāo bèi mèimei chīwán le.
The cake was eaten up by my sister.
verb + 完
门被风关上了。
Mén bèi fēng guānshàng le.
The door was closed by the wind.
the doer can be a thing
3. Share a tip
被 puts the affected thing first. You can drop the doer: 蛋糕被吃完了 also works.