Oh, oh!! I know that one!
In Sigil, this is simplicity itself. For every Chapter Heading, e.g., if you have:
"Chapter One"
Just highlight the Chapter One text and select "Heading 1" from the drop-down list on the left-hand side of the menu box. For sub-chapters, if you have them, choose Heading 2, and so on.
If you have an ACT structure--which I do for many clients--I use Heading 1 for the Acts (Act I, Act II, etc.) and Heading 2 for the Chapters. You can also do this in codeview, if you wish, in the html, which I actually how I do it, but everyone has their own way of working.
You can also use a spiffy feature of Sigil that I particulary like and end up using surprisingly often--you can create a TOC item for something that does not have text, like an illustration, by inserting a header on the page (you have to do this in codeview), thusly:
Code:
<h3 title="Illustration: Map of Disneyland"></h3>
or wrap the same header tag around an image. The text contained within the title attribute, in this case,
Map of Disneyland, will show up in the Table of Contents.
You can then view the TOC in the TOC editor. You can select items to omit by deselecting them (scroll to the right in the TOC editor where you'll see the checkmarks--uncheck anything you do not want to include).
You can start out getting a feel for this by just doing one Chapter header, then look at the TOC Editor to see it building; it'll give you a good idea of what's going on and help you build your TOC.
n.b.--you'll probably have to restyle your headers in your CSS, if you have not already included them in that file; be prepared for them to go wonky if the CSS isn't already there, so don't panic if they are suddenly Arial 14 or whatever.
I hope that helped!!
Hitch