Drupal Instapage module ported to Drupal 8
Iztok was original developer of Drupal 7 module, which later had couple of commits from my side. All the functionalities are the same so nothing shocking from feature side of the module. You still have to create account at Instapage first (if you don't already own one). Than you have to install drupal module on your site the standard way, connect your page to your Instapage account and from there you can assign url on which you would like to pull content from Instapage.
There are two screencasts with instructions. One from Iztok (https://www.youtube.com/watch?v=kp372BOLp5E) and the other from guys at Instapage (https://help.instapage.com/hc/en-us/articles/207038297-Publishing-Your-Page-to-Drupal). They are both for Drupal 7 version, but not much has changed and you should be able to achieve expected result with Drupal 8 as well.