In the Settings tab, under the Personal subtab, you can adjust your weekly availability. Click the plus symbol, and select the date that this availability request would become active.
A manager will need to approve this availability adjustment, and you can choose which manager should receive this availability request from the drop-down; or choose to simply send it to the entire management team. Since this does need to be approved, typing in a reason is always a good idea.
Then simply drag your cursor over the times you will not be available during the week. Gray sections signify you are not available.
When finished, click Submit Availability.
You will be notified when this availability adjustment has been approved or denied. You can always come in and cancel it, if needed.