my externals are downloaded but idk how to install them, how to do that?

didi you download them within pd via "help"->"find externals"? then they should be already installed. start a new patch in pd via "file"->"new" and in the patch window use "put"->"object" and type in the name of an external you downloaded.

have a look at http://booki.flossmanuals.net/pure-data/ for first steps.