Does anyone know how to add snow to map

Is there any guide for doing that, or can someone explain how? Thank you.

You can just apply a snow texture to the floor if you want snowy ground.

What kind of snow? Do you want snow on top of objects, like a snowy roof? Most Valve maps use a model for that, specifically made for that exact map.

Almost as a map covered in snow, so it would snow covered grounds and rooftops and some objects.