1. Explain it simply
第 turns a plain number into an ordinal: 一 (one) becomes 第一 (first), 三 (three) becomes 第三 (third).
The order is 第 + number + measure word + noun, for example 第三本书 (the third book).
第 attaches to the NUMBER, not to the measure word: it is 第一个, never 一第个 or 第个一.
Common set phrases: 第一次 (the first time) and 第一天 (the first day).
2. Show examples
这是我第一次来中国。
Zhè shì wǒ dì yī cì lái Zhōngguó.
This is my first time coming to China.
第一次 = first time
他是我的第一个朋友。
Tā shì wǒ de dì yī gè péngyou.
He is my first friend.
第 + number + 个 + noun
这是第三本书。
Zhè shì dì sān běn shū.
This is the third book.
第三本书
3. Share a tip
Always put 第 right in front of the number: 第 + number + measure word + noun. Learn 第一次 (first time) and 第一天 (first day) as ready-made phrases.