1. Explain it simply
在 (zài) means 'at/in/on'. Put 在 after the subject, then add the place word (家, 学校, 桌子上, 杯子里...).
Combine 在 with location words like 里 (inside), 上 (on top), 下 (under), 前面 (in front) to be more specific.
2. Show examples
我在学校。
Wǒ zài xuéxiào.
I am at school.
Subject + 在 + place
水在杯子里。
Shuǐ zài bēizi lǐ.
Water is in the cup.
Location word 里
狗在桌子下。
Gǒu zài zhuōzi xià.
The dog is under the table.
Location word 下
3. Share a tip
Think of 在 as the glue that links the subject to a location or direction word. If you can point to the place, you can drop 在 + that place into your sentence.