Adding a lesson in lesson grid

1. Select a subject and class. You can either click or use arrow keys:
2. Type the number of lessons per week into the selected cell:

Thats it!

The lesson is shown in the pink color because you have not yet assigned a teacher.

3. Once you know you can assign a teacher that will teach this lesson. Simply select the lesson and type first letter of this teacher or click into the combobox:

Timetables online
In timetbales online it works in similar way.

Check this article for various formats that can be used to specify double lessons, 2+2+1 lessons etc:
Count per week - format