Create a work
Open the Works screen
From the sidebar, go to Works.
Start a new work
Click New to open the work form.
Fill in the details
Enter at least a title and composer. Add any other metadata you have — you can always edit it later.
Save
Save the work. It now appears in your collection's list.
You do not need every field up front. Create the work with the essentials, then
upload a score and refine the metadata over time.