Click on an the reservation in the timeline ("All Rental Items"), and the following dialog will open with the reservation details:

Click the "Show borrow slip" button, and the borrow slip for this reservation will open.

You can print this borrow slip directly. It can be signed by the borrower when picking up the device.

Incidentally, all devices booked by the borrower are listed here. Click on one of the devices in the borrow slip, and it will be removed from the list (visually only).