From Home
Open Home, switch to the Docs tab, and choose New Doc. In the prompt, type a title or leave it blank — new Docs start as Untitled until you name them.
Inside a project
Open the project and use New Doc again on its Docs tab. Those Docs are descendants of the project. They inherit share access from the project you granted.
Find it later
Island Projects lists Recent and Favorites. Use the house button there to return to Home. Search from the island or the preheader with Search documents….
Cover and description
Click the cover placeholder to add an attachment. Open Properties to write a description in Add a description…. Neither is required.