Install a local development stack.
Create 2 development sites with 2 databases. I recommend to easily compare without swapping branches:
d8database to do your development against
d8clean.dev(optional) with a
d8cleandatabase to compare your conversion code against the latest core.
Clone Drupal 8 with git:
(run these commands from within your respective web directories)
git clone --recursive --branch 8.x http://git.drupal.org/project/drupal.git your/d8/webroot/path cd your/d8/webroot/path
Referenced from: http://drupalmentoring.org/sprint