If you are not a CZU student (e.g. you are a student of a different university in Prague, you are a boyfriend/girlfriend/friend of a CZU student, etc.) you'll have to make a registration at https://iskam.czu.cz/Registrace/registraceCZU.
Once you receive credentials (login and password) for ISKAM, you can follow these steps:
1. log in by using the credentials given
2. go to Accommodation -> Booking
3. Create a new booking