Create drupal-project with --no-install option:

    composer create-project drupal-composer/drupal-project:8.x-dev my_site_name_dir --no-interaction --no-install

Change directories to my_site_name_dir and edit the version contraints in the composer.json file:

    "require": {
        ...
        "drupal/core": "8.6.*",
        ...
    },

Also for 

    "require-dev": {
        "webflo/drupal-core-require-dev": "8.6.*"
    },

> webflo/drupal-core-require-dev provides the require-dev dependencies
> of drupal/core as a standalone package. It follows the same release
> cycle and versioning scheme as Drupal core. You should use the same
> version constraint for it as you use for Drupal core.

Run `composer install`

------------------------

### Warning

Since answering the question the de-facto standard for installing Drupal via composer has changed from drupal-composer/drupal-project to drupal/recommended-project. Now you can append the version directly to the package. See [Vinod Singh's answer][1].


  [1]: https://drupal.stackexchange.com/a/294656