1) Be sure to have the cPanel Remote Access Key generated (if it isn't already). This is needed to use the XMLAPI of WHM. Instructions on how to generate this key are available here.
2) A valid EUF license key, which you can obtain on this page: https://www.unixy.net/secure/cart.php?gid=9 . Be sure to pick the matching license. So for the paid cPanel Varnish license, pick the paid End User Features license. For the trial cPanel Varnish license pick the trial End User Features license
3) Navigate to WHM -> UNIXy.Net cPanel Varnish -> End User Features -> Enter the End User Features license and hit update
4) Enable the End User Features by clicking on Go