Panopoly is best installed from scratch, but it also can be installed on an existing site! The awesomeness of this experience will depend on how your existing site is setup, but this is a great way to get up and running with Panopoly today. In order to get this running, you need to do two things:
Install Required Contributed Modules and Libraries
Panopoly requires several contributed modules and external libraries to function correctly, often with specific patches necessary to make everything work together magically. These modules and libraries must be installed on your existing site in order to use the power of Panopoly.
The recommended way to get these modules is to download the latest version of Panopoly and run drush dl to properly download the correct dependencies. This can be done with Drush by drush dl panopoly or through direct download of the latest Panopoly release.
After you download the Panopoly package you will need to update your site with the themes, modules, and libraries bundled with Panopoly which is found in the downloaded file at profiles/panopoly.
Enable the Panopoly Magic
Panopoly is powered by a number of different apps that provided specific functionality to your site. At a baseline, you should enable the "foundational" modules of Panopoly Admin, Panopoly Core, Panopoly Images, Panopoly Magic, Panopoly Theme, Panopoly Users, and Panopoly Widgets. You can also enable any relevant "toolset" modules you might need which include Panopoly Pages, Panopoly Search, and Panopoly WYSIWYG.
drush pm-enable panopoly_admin panopoly_core panopoly_images panopoly_magic panopoly_theme panopoly_users panopoly_widgets panopoly_pages panopoly_search panopoly_wysiwyg
If the Panopoly's modules were not detected, that means you may need to set Panopoly as your installation profile.
Here are some modified steps from this article: http://davehall.com.au/blog/dave/2012/09/12/switching-installation-profi... that may work for you.
$ drush vset -y install_profile my_profile
An alternative way of doing this is by directly manipulating the database. You can run the following SQL on your Drupal database to switch installation profiles:
UPDATE variable SET value = 'my_profile' WHERE name = 'install_profile';
-- Clear the cache using MySQL only syntax, when DB caching is used.
Also you should:
echo "UPDATE system s SET schema_version = 0 WHERE s.name = 'my_profile'" | drush sqlc && drush cc all
It can also be a good idea to download and enable the Panopoly Demo module just to make sure things are looking sharp.
drush dl panopoly_demo
drush pm-enable panopoly_demo
Resolve Any Functionality Conflicts and Remove Duplicate Modules
Panopoly does make a number of assumptions about your site and does add/configure certain things such as a search override page, WYSIWYG, text formats, Panels and Chaos Tools settings, and more! Your custom site will may have conflicts here and those would need to be handled by a case by case basis. However, since each of the Panopoly Foundational Apps is properly featurized it's possible to revert the features to force the settings Panopoly requires.
Panopoly also bundles a number of contributed modules which have specific versions and patches that are required. In order to get this working, simply remove any module from sites/all/modules or sites/sitename/modules that are duplicated in the profiles/panopoly/modules/contrib directory.
Enhance Site Functionality with Panopoly Development Patterns
Once you have the required Panopoly apps enabled and your site running Panopoly, it is important to enhance your existing site to take advantage of Panopoly development patterns (i.e. switch to using Panopoly's responsive layouts, convert blocks to panel panes) which are outlined in the documentation on developing sites with Panopoly.