First thing you need to do is to install and activate the Last Viewed Posts plugin. After activating the plugin, go to Appearance » Widgets. Drag and drop Last Viewed Posts widget to your sidebar and it will immediately start showing posts a user last viewed.
/* Here are some parameters you may want to change: */ $zg_cookie_expire = 360; // After how many days should the cookie expire? Default is 360. $zg_number_of_posts = 10; // How many posts should be displayed in the list? Default is 10. $zg_recognize_pages = true; // Should pages to be recognized and listed? Default is true.