Open My shifts in the Portal sidebar. It lists your published shifts for this fortnight. Move to other weeks with the arrows. Roster shows the published roster of the locations you belong to.
You are told when a week you are on is published, and when one of your shifts changes. Your shifts also appear in My calendar and in your phone's calendar if you subscribed to your feed. A phone calendar refreshes on its own schedule, so a last-minute change reaches you by notification first.
Swap a shift
Offer puts one of your shifts up for others to claim. It stays yours until someone's claim is agreed.
Give to hands it to a named colleague at the same location. They accept first.
Shifts you could take lists open shifts, and shifts others offered, that you could work. Choose Claim.
If your workspace wants it, a manager approves the change before the shift moves. Your requests and where each one stands are listed below your shifts.
When you cannot work
Under When you can't work, add the dates you are unavailable: all day or between two times, once or every week. There is no reason field, because your manager doesn't need one. If you are already rostered then, you are asked to give the shift away instead.