first you have to download the mod, (look at the gusanos forum,) when you have downloaded the zip/rar file, unzip the map into te main folder of gusanos (the one where the maps default and screenshots already are).

now it is installed, thats it if you want to play it, all you have to do is to type in the console: game X (x stands for the mods map name)

if you intall a newer version, delete the older map first, so don't overwrite it, that can cause trouble