To create a sub-project, please click on the "Add project" link below your project list and while you see the project name input field opened, you can use the icons on the right to indent/unindent it. Alternatively, you can use Ctrl+RightArrow and Ctrl+LeftArrow.
You can convert any existing project (except for the Inbox) to a sub-project by right-clicking on it and choosing "Edit project" from the context menu to open the input field and then indent it.
Is there any more information about how this works? Indenting makes a project a subproject? Also, is there a guide available for how to use ToDoist? I've watched videos but they don't really get into the details.
That is pretty much the best description. You can check out Todoist.com/help and our YouTube. https://www.youtube.com/user/DoistApps