Ever since it supported devices, iTunes has had a "sync" button, which seems to have vanished from version 11. Where did it go? How do I sync my iOS devices with iTunes now?
The sync button is on the bottom right of the iTunes window when you are viewing a device.
Connect a device or two, and you will see controls to click in the top right to view the iOS device. Also option-command-s will toggle the traditional sidebar - now with more color than (immediately) before.
Selecting the device will bring up a mostly familiar interface. You can also search on sync in the iTunes help menu. This update has changed the default views to look flatter and focus more directly on one main view, but menu commands, key commands and buttons are there to get to sync settings as usual.