How do you download maps into Portal? I know this probably sounds stupid, but usually when you download maps for games, you have to save them to a specific location, and I haven't been able to find any sites that explain where to save them to. When I search it on google, I usually find just maps. ^^;;

Try downloading a few maps and reading the readmes. They should have information about that, hopefully you can find one that will make this clear. =\

Some maps come in .bmz format and then it is just enough to open them in Portal and they will install themselves into the Bonus maps menu.
(like my map)

but whatz aboutz ze non bmz? .bsp and .rar and such???

Again, read the readme files included (if any).

BSP files are plain and simple maps, they belong in your /maps/ directory.

RAR files are compressed files, download WinRar to extract them - and read the readme!