When you choose to create a new widget, you'll get two options: either use a common template, as shown below:

Or you can use one of the templates you have created (maybe you love the widget you built previously!):

Now that you're in the creation screen, here's what you'll see: 

From there, you can change your mind and the type of the widget if you're not satisfied with the one you chose. You can name your widget afterward, and choose the currency for it. By default, it will inherit the currency of your ad account. 

Two things here: selecting the Period would supplant the Time Range selected for the whole Dashboard. If you don't add one, the widget will inherit the Dashboard's.
The Granularity is the level of detail your widget is going to be displayed (Daily, Weekly, Monthly or Yearly reporting).

The Key Metrics part will ask you to select the data on which you want to have your report on. Adding the objective will make the widget clearer, telling you when you're over or under your target. Feel free to add several Key Metrics, but remember that the more you add, the more cluttered the chart will be. 

Tip: If you want to add several metrics at the same time, select them all in a row, and you will see a button appearing suggesting to add several metrics all at once ;)

Grouping is a great way to report on several ad accounts and/or campaigns at the same time. Simply select the common criteria you want your campaigns to be grouped by, and your widget will be built around it!
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You can also add Facebook Breakdowns in your widgets to get an even more in-depth view of your data! If you want to learn more, you can read up here!

Warning:  Pie Chart, Tops, and Trees will need you to create a grouping for the widget to display. 

Finally, you can put filters on your widgets, so that you'll only see the necessary data!
You can apply them on the Ad Account level too, in case you want to keep a specific widget for a specific account for instance.
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Once you've created your widget, there will be a menu offering a few other options: 

Edit simply takes you back to the Widget Creation screen. 

Display Mode will let you switch from Normal mode (usually smaller), and maximized, which will take up more space on your dashboard. For some Widget Formats, this will change their aspect completely.
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​Minimized (50% width): 

Maximized (100% width):

Export will let you download as an Excel the contents of the widget you created. For instance, if you export this widget:  

You will get an Excel like this:

Duplicate will copy your widget and add it to your dashboard.
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​Save as Template will save the widget you created for future use. Next time you will click on the Create a Widget button, you will have another option, like this:

Finally, Delete will get rid of the widget in your current dashboard.