xili-postinpost provides a triple toolkit to insert post(s) everywhere in webpage. Template tag function, shortcode and widget are available.
Upload the xili-postinpost plugin to your blog, Activate it. Go to settings.
If you want to use widget, go to Widgets menu of Appearance menu.
To use shortcode inside post's content, refer to examples provided in these posts here.
To use core functions of plugin, as developer, refer directly to code source before inserting (and echoing result) of the function in your theme.