The most common way to say it is:

To do it on the quiet

Anabel: Can you give your brother driving lessons before getting the driving license in the UK?
Norman: yes I can, as long as the car is insuranced, etc.
Anabel: wowww, we can’t do that here. However, there are some people who do it on the quiet to save some money, since driving lessons are expensive.

For more phrases like this:  Most used everyday English phrases
For idioms: Idioms
For podcasts: Podcasts

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