There are five tabs on the upper right corner in the main window: Documents, Projects, Customers, Tasks and Layout. The first three tabs display a document list which will be filtered by the following criterions:
Left sidebar: Smart folders, projects, customers
Filter bar: Paid and pending invoices. Invoiced and not invoiced quotations
Click the plus button at the bottom of the document list to open the context menu. Choose the desired document type (invoice or quotation) which you want to add. The document will be added with the following rules
If you are in the projectview (left sidebar contains a list of your projects), the document will be added to the selected project with the main customer of the selected project.
If you are in the customerview (left sidebar contains a list of your customers), the document will be assigned to the selected customer without a project.
If you are in the documentview (left sidebar contains a list of your smart folders), the document will be created without a customer and project, though you can set a customer later.