Download and install QtHelp SDK if you haven't already done so.
Select Main Menu->Tools->Options and enter "Path to QtSDK BIN folder" where qthelpgenerator.exe is located.
Select "HTML-based templates" and check there's a template for QtHelp. Helpinator's default.html is QtHelp-compatible.
Click "Add template" if you are not satisfied with default template.
Select templates that have "For: QTHELP" label.
Select root node in the project tree view and browse to "Templates" tab, then select template for QtHelp from the drop-down list or leave default.html.
Now you can click "Compile QtHelp" on the main tool bar to compile QtHelp help file.