Here is the guide I am reading to customized Horizon. Hope it will help you too. http://docs.openstack.org/developer/h...

I can just recommend you to take a look at the horizon ui design as I asume you are a ui man. take a look at /var/lib/openstack-dashboard. also refer to the DEVELOPER guide in openstack website docs button

These are the few blogs written simply to help you out, Other then that as @jtaguinerd mentioned that openstack official documentation is also a very good source. Let me mention some of helpful links other then official guide. see: building on horizon and this blog. Read this thread where a user has tried to add a panel. He explained all the steps he followed.