Unless you have managed to arrange permanent housing before leaving for Leuven, you need to book guesthousing for about one week.
Once in Leuven and settled in your temporary guestroom, you should immediately go to the KU Leuven Housing Service. Always bring your LETTER OF ADMISSION to the Housing Service! Without admission letter the housing officer can not give the proper housing advice.
At the Housing Office you will find the necessary information to guide you through the procedure to find permanent housing. Finding a place to stay can very well take up to a week.