Install Development Environment CiviCrm for Drupal 8
UPDATE: You can use this method but instead of a version 5.x in the composer command specify dev-master.

You can also use buildkit with e.g. target d8rec-clean. See https://github.com/civicrm/civicrm-drupal-8/pull/37



OLDER:

Here's what I've been doing. I don't think it's the ideal solution and am curious if anyone else has a better way.


Use the roundearth installer as normal to install the latest "numbered" version. (https://gitlab.com/roundearth/drupal-civicrm-project)
Hack the vendor/roundearth/civicrm-composer-plugin/src/Handler.php file so that:

(a) getCivicrmCoreVersion() returns 'NIGHTLY'

(b) addMissingCivicrmFiles() uses https://download.civicrm.org/latest/civicrm-NIGHTLY-drupal.tar.gz as the civicrm_archive_url.
Delete vendor/civicrm/civicrm-core
Run composer require civicrm/civicrm-core:dev-master civicrm/civicrm-drupal-8:dev-master