How to install a mod or map?

How to install a map? 

If the downloaded file has a "gro" extension, place it in the root folder of the game (default path "C:\Program Files\Steam\steamapps\common\Serious Sam The Second Encounter"). In the case of an archive (zip, rar, 7z, etc.), it must be unpacked into the root folder of the game.
In rare cases, a map may be created for a specific mod. In this instance, the .gro file containing the map should be placed in the mod's root folder, for example: C:\Program Files\Steam\steamapps\common\Serious Sam The Second Encounter\Mods\SVSE30

How to install a modification? 

Unpack the .zip file in the "Mods" folder of the game (default path "C:\Program Files\Steam\steamapps\common\Serious Sam The Second Encounter\Mods").

Recommended: At launch, it is recommended to install the standard control configuration for your player profile, as mods may have special buttons that are not in the original game. This can be done by going to Options → Players and Controls → Customize controls → Load Predefined Settings → Default controls.