1. Explain it simply
A one-syllable verb A doubles to AA: 看 becomes 看看. You may also say A一A: 看一看.
A two-syllable verb AB doubles to ABAB: 运动 becomes 运动运动. Never split it into AABB.
Doubling adds a soft, relaxed feeling, like 'have a little look' instead of a plain command.
2. Show examples
你看看这本书。
Nǐ kànkan zhè běn shū.
You take a look at this book.
看 to 看看
让我想想。
Ràng wǒ xiǎngxiang.
Let me think a moment.
想 to 想想
我们出去运动运动吧。
Wǒmen chūqù yùndòngyùndòng ba.
Let's go out and exercise a bit.
运动 to 运动运动 (ABAB)
3. Share a tip
Two-syllable verbs repeat as a whole block (ABAB), like 运动运动, never as 运运动动. Only one-syllable verbs can take the 一 form, as in 看一看.