Click on the tab "Resource booking" and then click on the tab "Library".
Now you get a list of rooms you can book.
Click on "Book" at an available time in a room, write your name in the window that pops up and click on "Book" in the window.
Confirm your booking! Note that you need to confirm your booking 15 minutes before - 15 minutes after your period begins. If the reservation is not confirmed, the reservation expires and the room becomes available for others.