Once you have an HTML or EPUB file, it’s time to load it into Sigil. Even if you have an EPUB file already you should follow these steps for a quick introduction to using Sigil.
If you are starting with an EPUB file, make a backup copy of the file before opening it in Sigil.
When you first start Sigil you will be shown a screen with several windows:
The Book Browser window on the left shows you all of the files in your EPUB, the Code View editing Window is in the center.
This is how Sigil might look if an EPUB was loaded:
You can open individual files in your book for editing by double-clicking on their names in the Book Browser.
See the User Interface chapter for more details.
To load your HTML or EPUB file into Sigil, click the Open icon 
or use the menu item : or the keyboard shortcut Ctrl+O
Select the file you want to open from your file manager, e.g. TestBook.html.
You will now see your HTML file(s) in the Book Browser window under the Text folder on the left. The file will be opened in the Code View editor in the middle, with its filename showing on the tab header:
Your HTML file is now part of an EPUB. From now on you are editing your EPUB and not just an HTML file. You may have noticed that when you opened your file in Sigil it created a couple of other files needed by the EPUB format, which you can see alongside your original document in the Book Browser.
If your file includes any images or stylesheets, etc., Sigil will attempt to load them as well and store them in the appropriate folder in the Book Browser.
When converting your file to EPUB or loading an EPUB Sigil may add some necessary details and organise your files into standard folder names.