想 + QW + 就 + QW (whatever you like)

You choose

• You take a visiting friend to dinner and want her to order anything she likes.
• you你nǐwant想xiǎngvariant of 吃吃chīwhat什么shén methen就jiùvariant of 吃吃chīwhat什么shén me。 — "Eat whatever you like."

want想xiǎng + QW + then就jiù + Verb + same QW

• (Subject) want想xiǎng + Verb + QW, then就jiù + Verb + same QW = do whatever / wherever / whenever you want.
• The same question word appears twice, and the same verb usually repeats: want想xiǎnggo去qùwhere哪儿nǎrthen就jiùgo去qùwhere哪儿nǎr.

you你nǐwant想xiǎnggo去qùwhere哪儿nǎrthen就jiùgo去qùthere哪儿nǎr。

With what什么shén me, where哪儿nǎr, what什么shén metime时候shí hòu, whoever谁shéi

• you你nǐwhat什么shén metime时候shí hòuto have time (to do sth)有空yǒu kòng，then就jiùwhat什么shén metime时候shí hòuto come来lái。 — "Come whenever you're free."
• whoever谁shéiwant想xiǎnggo去qùwhoever谁shéigo去qù。 — "Whoever wants to go can go."

whoever谁shéiwants想xiǎnggo去qùthat person谁shéigoes去qù。

Same word twice

Versus as one wishes随便suí biàn

• as one wishes随便suí biàn = "as you like" on its own: A: variant of 吃吃chīwhat什么shén me？ B: as one wishes随便suí biàn。
• The QW…QW pattern says exactly what is free to choose: want想xiǎngbuy买mǎihow many多少duō shaothen就jiùbuy买mǎihow many多少duō shao — "Buy as many as you want."

want想xiǎngbuy买mǎihow many多少duō shaothen就jiùbuy买mǎithat many多少duō shao。

Recap

Read the full reference: HSK 3 grammar — Basic sentence patterns.