How can I disable multiple desktops in Sierra?

You don't have to have multiple desktops.

Open Mission Control, by pressing the F3 key or swiping up with three or four fingers.

Move the cursor to the area on the top where you see the desktops.

Click the X to close each of them except the first. All your windows will be rearranged on the remaining desktop.

There is no way to change how full screen operates. If you would like the window to be bigger, but remain in the same Desktop, try holding down the option/alt key while clicking the green dot in the upper left hand corner.

As per this article macOS Sierra: Work in multiple spaces

Delete a space

1. Enter Mission Control, then move the pointer to the top edge of the screen to show the Spaces bar.
2. Place the pointer over the space you want to delete, then click the Delete button that appears.If the space contains open windows, they are moved to another space.

You can quickly stop using an app in full screen or Split View by moving the pointer over the thumbnail in the Space bar, then clicking the Exit button that appears.

When you put an application in fullscreen mode, it gains its own desktop. That is how it works.

Command-tab is your friend. Fullscreen or not, if you want to drag from an application to another, just start to drag, don't let go of the trackpad or mouse button, press command-tab to switch, then drop in to the other application.

Works beautifully.

Not sure what you want to work differently.

Command-tab switches between applications, whereas command-` switches between windows of the same application. It has always been like that; the only thing that complicates matters in recent versions is fullscreen, especially if you have different windows of the same application, where some are fullscreen and some not.

With fullscreen, as with multiple desktops, you can use control-right arrow and control-left arrow to switch.

Of course every user has personal preferences on how to work.

I prefer not to use fullscreen applications, and have only one desktop.

As I am writing this, the frontmost Safari window takes up most of the screen (but NOT fullscreen, still have the menu bar).

I can easily switch to Mail, or whatever application I need (usually running about ten of them).

Command-tab is my friend ;-)

OverHeadNotDuck wrote:

What is the keyboard shortcut for navigating between spaces/desktops in this situation: I have 3 word windows open, one of them is maximized into its own space/desktop. command-tab brings me to one the word windows that is in the same desktop, command-' navigates between the two smaller word windows.

Control-left arrow and control-right arrow allow you to switch desktops to the left and right; and control-1, control-2 etc let you switch to a specific desktop.

OverHeadNotDuck wrote:

I´m also having trouble using fullscreen function with multiple screens. When I fullscreen a window or video, my second screen goes black. Any tips?

Yours is a different situation.

I assume you only see the menu bar on the main display.

You can change both these things by going to System Preferences->Mission Control.

CHECK the box that says "Displays have separate Spaces".

Then log out and log back in.