Here’s an example Google document about the City of York.
At first glance it looks like it’s got a clear title, subtitle and headings, but if I enable the document outline panel you can see that the only part of the structure it has identified is the title.
To be fair, the outline panel can pick up the structure if you start changing the font size for headings, like this [outline changes when a font size is applied] but that’s not going to be a help to other tools that need to identify structure correctly in order to work and will not help produce accessible documents.
For these you must use heading styles.
So let’s do it properly and define the title, subtitle and headings using the styles.
I’ll remove any direct formatting and apply the appropriate style and as I do this you can see the outline developing in the side panel.
That’s the whole document done.
But what if I don’t like how it looks?
That’s simple - redefine the styles.
Select an instance of a style - I’m choosing some normal text here and make the changes.
I’m changing the font, but I’ll also change the line spacing and space after paragraph using this custom dialogue box.
1.6 line-spacing, and some space before and after.
Then, with the instance still selected, choose to update the style to match.
I would do this for all the styles I’ve used.
If I decide I'd like to use these styles for all new documents, one of the Options in the style drop-down is to set these as my default styles.