Drush makes it very simple to create a subtheme. For this you need to download, install, and enable the Omega Tools module first (this should be done anyway as Omega Tools has features outside Drush such as theme exporting). Once you have Omega Tools and Drush installed and configured, perform the following steps.
Create a subtheme with Drush
From a shell console in your site’s root directory (not being in your site root can cause problems in the Drupal installation) type
drush omega-subtheme mysubthemename where mysubthemename is the name you want to give your custom subtheme. Don’t use any dashes or spaces in the subtheme name, or make sure to enclose the subtheme name in quotes. If you are running a multi-site setup and you want to create a new subtheme in a particular site's themes directory, navigate to sites/mysite and execute the command
drush omega-subtheme "My Theme" --destination=mysite.
Your subtheme will by default be based on the Omega HTML5 starterkit.
To base your subtheme on the Omega XHTML starterkit use the option "starterkit":
drush omega-subtheme mysubthemename --starterkit=starterkit_omega_xhtml
For all other options enter:
drush help omega-subtheme
Check the theme folder and names of CSS files
Once the subtheme is created successfully you will see your new directory under /sites/all/themes/mysubthemename.
/sites/all/themes/mysubthemename/css you will find multiple css files. Check to make sure the name you gave your subtheme is used to name the css files. An example would be mysubthemename-alpha-default.css . If you see "YOURTHEMENAME" instead of the name for your subtheme, be sure to replace it with the correct name of your subtheme.
Begin using your subtheme
You are now ready to begin using your subtheme Go to
admin/appearance and you will find your newly created subtheme ready to be used. Enable your subtheme only. Remember, there is no need to enable the Alpha and Omega core themes.