To import your posts from your self-hosted Ghost instance, you first have to download your ghost.db from your server and run:

$ ruby -rubygems -e 'require "jekyll-import";{
      "dbfile"   => "/path/to/your/ghost.db"

There are no required fields. dbfile defaults to "ghost.db".

If you have a Ghost backup file, consider using another tool called jekyll_ghost_importer to import your content. It is a separate gem and docs can be found at the link provided.