To create an assignment, navigate to Course Setup > Readings and add a textbook to your course, or upload one or more documents. Then click once on the document in the Documents panel, and click Assign:
From there you can select one or more sections from the book or document to assign, and set a due date and other options.
You can set a name for your assignment, or allow Perusall to name it based on the document and page/section range you assign. If you decide to name the assignment, ensure the name is unique. (Perusall assumes that assignments with the same name are being assigned to different students and will merge those assignments into a single column in your Gradebook, which will lead to unexpected results if that is not your intention.)
Once you do this, the new assignment will appear in the Assignments panel:
To edit an assignment, click once to highlight the assignment in the Assignments panel, and then click the gear icon in the toolbar:
To delete an assignment, click once to highlight the assignment in the Assignments panel, and then click the trash can icon in the toolbar: