Log into Canvas and go to the assignment in question, click Edit
Scroll to the bottom of the Edit page to the Assign To section. Click +Add
Clicking +Add will creat an additional box where you can choose a specific student (or multiple students) who will have a separate time range than the rest of the class.
You should now have two boxes with individual time ranges. You can create more by clicking +Add at the bottom as needed or assign an entire section one time range and a separate one to another section.