On the top icon toolbar, there's a button decorated with an ornate 'Ch'.
This is what you will use to split your file into its component chapters again.
Just scroll down to where you want a new chapter to begin, and press the 'Ch' button. Each new section will form a new file.
NB Since writing this, Sigil has received another upgrade. The latest version hasn't got the ornate 'Ch'. But the same function exists under another name.
Click the icon that I've circled to create a chapter break at your cursor. I'm going to leave both versions here, just in case anyone is running the old one.
I don't think it matters what the file names are called. But I'm a bit pedantic over these things. I go through each file, right-click it and select 'rename'. Then I list my chapters 01-10.
Of far more importance is what you do next. This will come into its own, not only for aesthetics, but in creating a table of contents.
- Open each chapter/section.
- Edit the top of the page to write a chapter heading.
- Take care to select a heading style from the pull-down menu on the left-hand side. (It doesn't matter whether you center it, justify it or leave it be, as long as you choose any of the heading styles.)