1. Explain it simply
几 means 'how many' and expects a small answer (usually under ten).
Put a measure word after it: 几 + measure + noun (几个人, 几本书).
Use 几点 for clock time and 几岁 for age.
2. Show examples
你家有几个人？
Nǐ jiā yǒu jǐ gè rén?
How many people are in your family?
几 + 个 + noun
你几点回来？
Nǐ jǐ diǎn huílái?
What time do you come back?
几点 for time
你弟弟几岁？
Nǐ dìdi jǐ suì?
How old is your younger brother?
几岁 for age
3. Share a tip
Don't forget the measure word: say 几个人, not 几人. For larger or unknown amounts, use 多少 instead.