In LayOut, right click on the menu bar–or toolbar. These terms can be confusing. (On my screen, it’s on the top.)
A pop-up appears, with the names of the toolbars you have displayed, an option to Lock toolbars and an option to Customize. Click that last option.
A dialog pops up, and those same toolbars you have displayed show up. On my screen, I have “Main Toolbar” and “Standard”. The Standard toolbar is where I have the commands: New, Open, Save (as icons) and Document Setup… Perhaps there is an icon for it, but I don’t really care.
Highlight Standard in this list, select the Commands tab.
If the default Category highlighted in the left pane is “All Commands” that’s fine, or you can speed things up by selecting the “File” category. In the right pane, scroll around until you find “Document Setup…”
Click on it, and DRAG it onto the Standard Toolbar until you see a thing that looks like a tall, skinny capital “I” (the kind with serifs). When you see that, and it’s in the place where you want the command displayed, release your mouse button and there it is.
You can add all kinds of useful (and useless or redundant or idiosyncratic) commands in this same way. You can even make new menus. You can do this until there isn’t any more screen real estate to draw on. Or you can get back to work and make documents.
Edited to add: DaveR beat me to the punch, and with so much more elegance. It’s important to note, though, than when dragging a command onto the tool bar, that I-shaped thing MUST appear, or nothing will happen, other than frustration on your part. (And, perhaps Dave’s because he’ll have to explain the obvious all over again. Helps to be REALLY well experienced in the software, unlike myself.)