Instapaper Friendly is a WordPress plugin for making your posts… Instapaper friendly. The plugin simply adds a div before your content that marks it as the content that Instapaper should use as described by their publisher guidelines.
To install, just unzip the contents of instapaper-friendly.zip into your wp-content/plugins directory. BAM! It’s even easier if you just grab it install it from the WordPress plugin installation thingy itself.
You can download the Instapaper Friendly at its WordPress Plugin page.