With the Relative Posts plugin widget you can show up on the sidebar a list of related articles with or without thumbnails very easy.
So you already have installed the plugin, now
-Go to the Appearance menu in WordPress and select 'Widgets'. -Drag and drop the widget 'Relative Post' to the Widget Area you want.
Option Description: 1. Title: Just write the title you'd like to show for the widget(the defoult title is "You Also Might Like..."). 2. Max Number Of Posts: The maximum number of relative articles that can been displayed(the default number is 5). 3. Use Thumbnails: Check if you want to display the feature image(thumbnail) of the post. Otherwise display only the title. 4. Post Title Length:Check and write the number of characters that the post title will be had(the default number is 50). Otherwise will display the full length of title.