1. Explain it simply
Put a measure word between a number and the noun. 个 (gè) is the all-purpose measure word for people, friends, apples, etc.
Use 本 (běn) for books/notebooks and 些 (xiē) for 'some' or 'a few'.
2. Show examples
我有三个朋友。
Wǒ yǒu sān ge péngyou.
I have three friends.
Number + 个 + noun
这本书很好。
Zhè běn shū hěn hǎo.
This book is very good.
本 for books
这些水果很多。
Zhèxiē shuǐguǒ hěn duō.
These fruits are many.
些 for an unspecified amount
3. Share a tip
If you're unsure which measure word to use, 个 is safe for most beginner situations. Learn special ones (like 本) with the nouns you see often.