"To make a brand new collection or edit an existing one, visit the Subscription Manager, which you can navigate to by clicking Manage subscriptions from the left side of your YouTube page. Click the 'Create new collection button'. Name your collection and select the channels you'd like to include in your collection," explains YouTube. Use the search feature to find the right channels.
Collections are placed at the top of the subscriptions list in the YouTube sidebar, so it's easy to find them. They have a small arrow button that lets you expand the collection and see each subscription. If a subscription is added to collection, you'll no longer see that subscription in the sidebar without expanding collections. It's a great way to organize your subscriptions.