A collection is the set of designs that you want to to get priced and/or produced. Collections can include a single item or dozens of designs.
Once you create a collection, you'll be able to add designs, submit for pricing, and ultimately send your order off to production.
Click on the team you'd like to create the collection with
Click "New Collection" and give your collection a name
Click "Create collection"
To add existing designs to your collection, simply browse to the design, click the ellipses, click "move", then choose your collection.