A *.dsc file is produced when you build a package with ["debuild"] - if you're in the source directory of a Debian package you can just invoke ["pdebuild"] and not worry about it though.

Sources are normally not installed. You can only install them if you know the package name. When installing the sources of a debian package, a package ending on .dsc (description) is downloaded along with an original ["tar ball"] and a debian specific ["diff"] (compressed). The .dsc will contain the name of the package, both, in its filename as well as content (after the Source: keyword).