To begin using a Marble Sandbox go to sandbox.marbleize.io and register your developer account for the desired network. Once registered you will be able to create a new Marble Group with your developer account set as the group manager. This will allow you to mint new digital items for your group.
Once items have been minted they can be customized like any normal Marble contract. By default, Marble Sandboxes will have all feaures installed that have been approved and merged into the developer branch on the official Marble Github page.