You can create a permanent assignment from Assignment entry.
Firstly you will need to create a Rate code for you Permanent assignments selecting the Rate Code Type as Permanent Fee:
To set up the assignment, select the Permanent option from the Assignment Entry screen and then Add new assignment .
Now enter the the assignment details. You can enter a fee or a percentage and then select the calculate fee for the system to generate the value. Once completed save the assignment.
When the assignment is saved the system will automatically generate the timesheet with just a billing element for the Fee amount.
The timesheet will now be available to invoice via the Create Sales Invoices and selecting Permanent fee.