There are a few different ways to create a reflowable EPUB.
One way is to use a dedicated EPUB creation tool like Adobe InDesign or Sigil. With these tools, you can create your content in HTML and CSS, and then package it all up into a single EPUB file.
Another way is to create your content in a word processor like Microsoft Word, and then use a tool like Calibre to convert it into an EPUB file.
Finally, you can also hand-code your content in HTML and CSS, and then use a tool like Calibre to package it all up into an EPUB file.
The main thing to keep in mind when creating a reflowable EPUB is that your content should be structured using HTML and CSS. This will ensure that your EPUB file will resize and reflow correctly on different devices and screen sizes.