Your timetable is "set" by the timetabling unit. However, you have the option to block off up to two specific time periods if there are alternative free classes (ie. if there is no alternative lecture, or it is full, then blocking will not assign you to the other class). Your timetable will then be set again. In this sense, your timetable has an element of customisation.
Note therefore, that it is very hard to obtain a timetable fully matching your wants. You may wish to try a good number of times to vary your timetable as full classes sometimes will be free again.
You will be able to do this customisation through MyUni, after the timetable has been issued after/on O-Week.
You can also customise your timetable through the Student Centre (to be assigned into a full class), but I think you need a reason (eg. a clash).