Start with a project on Paper or jump right into "the mix" (sorry, couldn't resist). After launching Paper, you'll see the familiar collection of notebooks. To access Mix, swipe down to push the notebooks away and reveal Mix's interface. Now there's a new row of projects to explore, starting with any you've shared, either from Paper or those you've found and edited while exploring.
I'm following several creators, and each is represented by a thumbnail. When I tap that image, I can zoom in on a particular project. The resulting screen shows the original creator's name, the date the project was uploaded, how many "hearts" it has earned (hearts are favorites) and an option to mark it as a favorite myself. But the real fun is browsing the remixes.
Swipe across the image to see the next version of it in the "pile." Each variation lists the creator's name as well as who inspired that particular iteration of the image. You can add to anyone of those images yourself by tapping the image to go full screen, and then tapping again to bring up the tools. If you like the changes you've made, confirm them and a new version is added to the pile for others to view and remix.
And hey, you say you've got on iPad? No problem! You can access Mix with a browser at mix.fiftythree.com. Use the responsive web app to monitor your stream and mark favorites.
The whole thing is a lot of fun and is clearly the next logical step for Paper. It's just as satisfying to use casually or with professional colleagues. It's one thing to have a digital art studio in your pocket, and quite another to share its contents with collaborators so easily. Mix is that digital art studio.