This plugin will add links at the bottom of every post to other similar posts. The plugin also lets you pick other WP blogs as partners.
In WP Admin visit Plugins (lower left of screen).
Click "Add New" button (top center of screen).
Search Term "BlogGlue Related Posts" (center of screen).
Click "Install Now" under "BlogGlue Related Posts Network Plugin" (first result).
Click "Activate Now" (center of screen).
Follow on screen setup instructions.