As any app for Confluence Cloud, you can install the app via the Confluence admin menu.
Log into your Confluence instance as an admin.
Click on “Apps > Manage apps” in the header menu.
Click on “Find new apps” on the right navigation.
Search for “Dialogs CSS for Confluence” and select it.
Click on “Try it free” to install the app.
Dialogs for Confluence provides a macro that allows you to add any Confluence content into a Confluence dialog. To make use of the macros click the “Insert” / “+” Button in the edit mode and select the “Dialogs for Confluence” macro. You can now enter your Confluence content as macro body and add a text for the button to open the dialog in the macro options. The macro options also provide settings to define the dialog size.