What is the best way to say this?

When one has a flat tire, it is necessary to put air in it.

I have to translate this, and my best guess is

Quand on a pneu à plat , il faut mettre de l'air dans ça

That doesn't really look right me at the end. Could someone check it? Merci

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Quand on a un pneu dégonflé, il faut (il nécessaire) mettre de l'air en dedans.

Mme