This documentation is not using the current rendering mechanism and is probably outdated. The extension maintainer should switch to the new system. Details on how to use the rendering mechanism can be found here.
This extension simply clears a little bit the final output of a page by using various optimization-methods. The possibilities you'll find under the reference in this user manual.
Install the extension and go (for example) to the Constant Editor of a template where you find all the possible options under “PLUGIN.SOURCEOPT” (see also reference below). Nothing else to do.
Note: The EXT:sourceopt process need server performance, because there are several search replace operations in the PHP logic of the extension. Please take care of this and monitor your server.