WordPress.org

1. You can download and install BuddyPress hovercards using the built in WordPress plugin installer. If you download BuddyPress Hovercards manually, upload the whole folder to "/wp-content/plugins/".
2. Activate the plugin through the 'Plugins' menu in WordPress

If you want to add a custom hovercard, or change the displayed fields, copy '/bp-hovercards/templates/hovercard.php' to the root of your (child) theme and edit it there to prevent it being overwritten at an update.