So ...

All you have to do is to install and activate the plugin.

Then, you can create a new post/page or edit an existing one, and you'll notice a newly added "Insert Button" item in the toolbar of your Classic Editor ...

... which will bring up a pop-up window where you can design your new button ...

First, you need to define the button text and to add the target URL:

Next, you can use the styling options to customize the text and background colors, the button type and the button size:

More than that, if you select the "Icon" tab, you'll be able to add an awesome icon before or after the text:

And that's all! When you are happy with your choices, simply click "Update" and the new button will be automatically inserted into your content. You'll be able to see your button as it would appear in your post/page, right inside the post/page editor. If you select the button text, you can use the editor's alignment features to align it to the center, left, or right. Double clicking on the button will bring up the Insert Button pop-up again ...

OK. Now let's see the result ...