So ...

Upon activation, go to Appearance => Widgets. As you'll notice, a new "Tag Cloud (Simple Tags)" widget has been added to your widget list:

Drag it into your sidebar area and expand it to see its settings:

All the options are very straightforward, allowing you to easily customize the number of tags you want to display, the font sizes, the colors, etc. What is important: in order to display your tags by popularity, you'll have to select the default "Counter" option for "Order by for display tags" and the default "DESC" option for "Order for display tags".

And of course, don’t forget to click on the "Save" button to store your settings ...

And that's it. You can now visit your website to see the plugin in action.

So, let's see the result ...