Starting from a blank popup

WP > Templates > Popups > Add new > [give it a name]. When shown choose a template option just close it.

Copying an existing popup design

WP > Templates > Popups > Add new. When shown choose a template option select ‘My Templates’

Select the existing popup > Insert

The original won’t be altered, you will just start off using its design.

Creating a popup that is displayed from a link / button

Saving the popup

Restrict this popup to being loaded only for the page it is being used on:

Add condition > Singular > Page > [enter the page name]

Opening it from the link / button

Link ‘Dynamic Tags’ icon > Actions > Popup > Open popup > [enter the popup name].