1. Explain it simply
正在 goes directly in front of the verb to mean 'is doing something right now'.
The pattern is Subject + 正在 + Verb (+ object) (+ 呢).
A final 呢 often reinforces that the action is still going on; 正在 must stay together as one word.
2. Show examples
妈妈正在准备午饭。
Māma zhèngzài zhǔnbèi wǔfàn.
Mom is preparing lunch.
正在 before the verb
我正在洗衣服呢。
Wǒ zhèngzài xǐ yīfu ne.
I'm washing clothes.
final 呢 reinforces it
你正在做什么呢？
Nǐ zhèngzài zuò shénme ne?
What are you doing right now?
question with 呢
3. Share a tip
Keep 正在 together right in front of the verb, never after it or at the end of the sentence - then a final 呢 makes it sound natural.