要 + Verb + 就 + Verb (if you're going to, do it right)

Five minutes of guitar a week

• Your friend wants to learn the guitar but only practises five minutes a week.
• if going to要yàoto learn学xuéthen就jiùgood好hǎogood好hǎoto learn学xué，don't别bié三天打鱼sān tiān dǎ yú两天晒网liǎng tiān shài wǎng。 — "If you're going to learn, learn properly — don't do it in fits and starts."

if going to要yào + Verb + then就jiù + Verb (+ properly)

• if going to要yào V then就jiù V = "if you're going to do it, do it (properly / now)": if going to要yàogo去qùthen就jiùgo去qù, if going to要yàobuy买mǎithen就jiùto ingratiate oneself买好mǎi hǎo(one)的de.
• The same verb repeats; the second often has a modifier: good好hǎogood好hǎoto learn学xué, early早zǎoa bit点儿diǎnrgo去qù.

if going to要yàogo去qùthen就jiùearly早zǎoa bit点儿diǎnrgo去qù。

Encouraging decisiveness

• if going to要yàobuy买mǎithen就jiùbuy买mǎiquality质量zhì liànggood好hǎo(one)的de。 — "If you're going to buy one, buy a good-quality one."
• as for要说yào shuōthen就jiùvariant of 说说shuōclear清楚qīng chǔ。 — "If you're going to say it, say it clearly."

if going to要yàobuy买mǎithen就jiùbuy买mǎiquality质量zhì liànggood好hǎo(one)的de。

Same verb, firm tone

if going to要yào…then就jiù… versus want to想xiǎng…then就jiù…

• want to想xiǎnggo去qùthen就jiùgo去qù = go if you want to (permission, freedom).
• if going to要yàogo去qùthen就jiùearly早zǎoa bit点儿diǎnrgo去qù = if you're going, go early (advice about doing it well).

want to想xiǎnggo去qùthen就jiùgo去qù吧ba。

Recap

Read the full reference: HSK 5 grammar — Sentence patterns.