1. Explain it simply
一 … 就 … links two actions: as soon as the first happens, the second follows.
Put 一 before the first verb and 就 before the second: 一 + V1 + 就 + V2.
Keep both halves - say 他一下课就回家了, not 他下课回家了.
The two actions share one subject at the front.
2. Show examples
他一下课就回家了。
Tā yī xiàkè jiù huíjiā le.
Sandhi:Tā yí xiàkè jiù huíjiā le.
As soon as he finishes class, he goes home.
quick result
我一回家就睡觉。
Wǒ yī huíjiā jiù shuìjiào.
Sandhi:Wǒ yì huíjiā jiù shuìjiào.
As soon as I get home, I sleep.
一 + V1 + 就 + V2
她一起床就喝水。
Tā yī qǐchuáng jiù hē shuǐ.
Sandhi:Tā yì qǐchuáng jiù hē shuǐ.
As soon as she gets up, she drinks water.
one subject
3. Share a tip
一 marks the trigger and 就 marks the quick result - you need both, and 一 here is read yī, not the number 'one'.