Use the roof tool - even a "flat" roof will have some slope.
Change the geometry method to 'Single plane' rather than 'Multi-plane'.
The use the 'Rectangular' construction (unless it is not a rectangular shape).
The parapet is just a wall.
As for a video specifically for this situation I don't think you will have much luck unless someone wants to make one for you.
But in the HELP menu you will see a link to the Archicad YouTube channel.
In there you should find videos for the roof tool - although most will be for the multi-plan roof.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10