Log into JustHost (not the WP admin on the client side but the admin on the server side)
Locate the leatherdiary-premium directory under /public_html/wp-content/themes
Locate the target subdomain (e.g. UnderTheHood)
Copy the leatherdiary-premium directory whole from /public_html/wp-content/themesto /public_html/<subdomain>/wp-content/themes (e.g. the final directory structure for UnderTheHood would look like public_html/UnderTheHood/wp-content/themes/leatherdiary-premium)
Now log into as WP admin on the subdomain side
Goto 'Appearance' on the left widget
Pick 'Themes' from there and then activate Leather Diary Premium