Need Help to write extension

Hello All,

Need a tutorial/help to get started on writing Libreoffice 
plugin/extension. Essentially, I would like to do the following in my 
plugin.

1. Open a file
2. Verify and convert the file from a format (for instance, decode a 
custom encoded file). If needed prompt the user for decode key with a 
dialog screen.
3. After editing, save with custom encoding.

I intend to use Java as programming language. I have referred to 
https://github.com/smehrbrodt/libreoffice-starter-extension


But, I have no clue where to start. Any help would be appreciated.

Thanks in advance.