Now that you have your EPUB in SIGIL, we need to split it into individual XHTML pages. This process is simple but tedious. It is also a little more complex to explain verbally, so I recommend you review the video as you work.
How to Convert EPUB to Kindle Digital Publishing in One Step
How can you convert an EPUB file to Kindle’s MOBI / AZW format? Actually you only need one file, the EPUB, and it will work for both.