You need drush version 6 installed, version 7 and 8 won't work
create drush alias
located at /vendor/drush/drush/aliases.drushrc.php
For testing we use a fresh subdomain before we roll out to live later on
$aliases['old_site_drupal6'] = array(
'root' => '/Sites/www.oldsite.com/htdocs',
'uri' => 'www.oldsite.com',
);
$aliases['new_site_drupal7'] = array(
'root' => '/Sites/d7.oldsite.com/htdocs',
'uri' => 'd7.oldsite.com',
'db-url' => 'mysql://root:root@localhost/new_site_d7_db',
);
Create 2 databases one for the old site, one for the new site.
Name one like your old database and the other one how you want to name your new database like new_site_d7_db