Using this thing-a-majig:
https://adweb.cis.mcmaster.ca /mtt/
If your changing a lot of things (which I don't recommend - cuz it'll be hard to get all of your options). Create a couple timetables for urself. So, if you don't have one option, you can still work things out with another. And remember, people will be dropping and picking up courses throughout the month till September. So, if you don't get what you want, be patient.