I will make the information on Sigil as good as I can for the version of Sigil that I am running now (version 0.3.4). Because Sigil is an open source project, new features and interfaces are likely to come out. If the information provided here does not seem to match up with the version of Sigil you are using, you may want to check the official Sigil site for help.
And now on to Sigil. This description assumes that you have already downloaded and installed the program; if you have not done this yet, please visit Self-Publishing an Ebook, Part 1: An Overview of the Programs for information on where to download this free program. Remember from the overview that Sigil is the program I use to edit and create .epub projects (or ebooks, as the finished products are called).
This is the starting screen when you first open the program (you can click the picture to view it larger):
Some of the basic elements you see on the screen are the buttons at the top. Here is an overview of some of the buttons on the top row of the screen, and what they do. There are some pretty basic buttons which duplicate the functions on the drop down menus at the top (i.e., going File -> Save vs. clicking the Save button) and are commonly found in many programs. If you are a writer, some of the buttons probably already look familiar to you, such as the Bold, Underline, Italicize, and Text Justification Buttons--these work exactly the same in Sigil as they do in your regular text editing program. When in Sigil, you can hover your mouse over a button if you're not sure what it does, and it will pop up some guiding text to tell you.
Some of the more unique buttons are explained below:
The three buttons on the left control the view. Because Sigil is a WYSIWYG editor, you have an option to view the text as you would in a regular word processor (the book button), as html code (the brackets), or in a split view mode that shows you both the finished text and the underlying code in side-by-side windows (the book with the brackets).
Onward to other features. On the left side of the main screen, you'll notice that there are various folders with names like "Text", "Styles", "Images", etc. Mostly, you probably won't touch these. They are doing what you would guess--the "Text" folder is storing all of the text chapters in your book, the "Images" folder is where the pictures are stored, the "Styles" folder has all of the fancy CSS markup that makes your book look formatted and stylish, etc.
It gets organized this way for the computer. Think of it this way: machines (and ereaders) like for things to come in boxes with a set of instructions on how to put it together. So, Sigil takes all of the stuff you are putting into your book and breaks it up into the boxes for you. It also creates the instructions for assembly (the CSS code) and includes it so that the ereader, computer, or other device knows how to put your book together correctly when someone wants to look at it.
The only folder you will likely use a lot in Sigil is the "Text" folder, because this is where you will double click to open up different chapters and sections of your book to edit. When you are adding an image or changing a style, you don't need to worry about the folder--just click the "Insert Image" button or format your text, and Sigil will update the folders for you.
There may come a time when a formatting issue gets particularly difficult. For example, let's say you put in a space between every paragraph in your book, but you really just wanted an indent at the start of each paragraph. In this case, you may need to change the view to look at the code (don't panic!) and open up your Style Sheet to make some changes. I am planning to cover the basics of tweaking CSS and HTML in a later post. Using CSS and HTML is worth it--if you end up making a formatting change that spans your entire book, and then you hate it, you probably won't want to go back through each of 300+ pages changing it back. Save your time for writing, not for click, delete, click, delete, click, delete, click... CSS can make document wide changes by changing it in one place.
That's all for now. Please feel free to contact me with questions/comments.