From the lessons' window
To schedule a lesson from the lessons' window you need two windows: the lessons' window and the timetable.
The "UnSched Prds" column indicates the number of unscheduled periods and also a hand showing that these periods may be scheduled manually.
You can use drag&drop to take hold of the hand in the "UnSched Prds" column and to drag it to the timetable where you can drop it.
From the timetable window
Alternatively, you can schedule lessons directly from the timetable window. The unscheduled periods are displayed next to the timetable by enlarging the window to the right or by clicking on the button < Resize the window>. These lessons can be scheduled with drag&drop.
Each individual step can be undone by using the <Undo changes> button in the toolbar of the timetable.