How does altimeter reading vary when departing from ISA conditions?
We set it to less than 1013 → it will indicate more than 0'


Above statement is incorrect for all scenarios.

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Remember, the anticlockwise rotation of the knob to set lower than 1013 also corresponds to anticlockwise rotation of the altimeter hands.

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Above image is what you're after.

Above the transition altitude (varies by location) you set standard setting regardless of the ISA deviation.

Below the transition altitude (TA) or when descending through the transition level (TL), you set whatever the ATC advises. From a pilot's perspective, it's that simple.

Unless you will be performing an approach in a cold area (usually below 0°C/32°F), then special altimetry equipment may be required. Requirements will be noted on the approach plate.



Related:


Wikipedia: Flight level
Skybrary: Altimeter Temperature Error Correction
How is non-standard OAT accounted for?