Log into your WordPress admin (i.e., your dashboard)
Go to Plugins->Add New
Type in "Highlight and Share"
Install and Activate the plugin
After activation, you can find the plugin's settings in the Dashboard under Settings->Highlight and Share.
Download the plugin zip file
Unzip the plugin zip in a place you can remember for the next steps
Log into your FTP program of choice for your website
Upload the unzipped folder into wp-content/plugins
After activation, you will be taken to the plugin's settings screen.
Log into your admin dashboard
Head to the plugins screen
Search for Highlight and Share.
Activate the plugin