A sandbox will be automatically created when you open a pull request on edx/edx-platform, if you belong to the Owners team on github. You can monitor the progress of the sandbox creation on the OpenCraft instance manager.
SSH to the instance, using your github username and the public SSH key from your github account. The keys are automatically refreshed from GitHub on every provisioning run - so if you need to use a different SSH key, update it on your GitHub account and restart the provisioning of your sandbox.