So, you have some geometry that was just created and you want to make a block out of it for your block library.  The simplest way to make the file is to use the WBLOCK command.  This command is no longer in the pull-down menus, but has been a vital tool for many years.

Type WB at the command line to enter the Write Block dialogue box.  The criteria is pretty straight forward.  A base point, select some objects, and a destination for the new file.  The other options are self explaining.

Don't forget that this is the only tool that will make dynamic blocks in your library directory as well.