On the Rental Unit tab, left-click on one of the units displayed in the rental unit grid, and right-click. This brings up the action menu. Select Invoice rental units, which opens the invoicing control screen. It is set up for 28 day in advance invoicing (see help on how to change setting).
Click Post invoice and the invoice for all the units will be displayed.
Click on the printer in the upper left corner and the invoice will print.