The Stacks plugin builds upon the SiteOrigin Page Builder plugin, which the theme has already included so there’s no need to download and activate that plugin separately.
The steps to use the feature is the same with SiteOrigin Page Builder plugin. Because of this, you can use the how-to videos on the plugin’s website: https://siteorigin.com/page-builder/documentation/
There is one major difference though: the SiteOrigin Page Builder plugin only changes the content area of you post/page. The Stacks add-on, however, uses the same SiteOrigin Page Builder interface but allows you to customize the entire layout of your post/page, except the header and footer.
To get started quickly, you can try replicating the standard Franz Josef post/page layout in the Page Builder by clicking on the “Layout” button, and then choosing one of the included prebuilt layout for Franz Josef. See the screenshot below to see what I mean.