Adding subtitles manually

I’m trying to add more boxes and instead of adding is deleting the boxes where I already wrote. I’m getting a very hard time to add subtitle manually. Also I am having lot of issues handling the  start and finish of each subtitle. Is asking me to remove or delete the box when i try to reduce the time and or is telling me that I have to follow the sequence of the first box when im already following it but did not let me proceed. Is there a tutorial for adding subtitles manually?