Upload the plugin folder, bwb-photosmash, to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
To automatically add a new gallery to a post, put this shortcode in the body of the post where you want it to show up: [photosmash]
To add a specific existing gallery to a post, use this shortcode (replacing ## with the gallery's id): [photosmash id=##]
Alternatively, you can set PhotoSmash to "auto-add" galleries to all post by updating the settings in the PhotoSmash options page in the Settings Admin menu.