Please check out this video: 
https://www.youtube.com/watch?v=WZ6fDhE-dBY

And here is a written guide: 
http://kodi.wiki/view/HOW-TO:Setup_subtitle_services


NOTE: some subtitles require login, please go to their website and register for free, and then input their login information in the subtitle configuration page. 



IMPORTANT: if you want another language than English Subtitle, please go to: 
Kodi -> System -> Video -> Subtitle -> and select languages you like as subtitle.