To access your General Availability, navigate to the red thumbs-up icon in the top-left of the screen.
On this page, you can use the table provided to select your Availability. To do so, click on the earliest hour you are available at and drag your mouse to the last hour of your Availability. Each available timeblock will be marked in green, with a thumbs-up icon, and each unavailable timeblock will be marked in red, with an O. At the top of the page, you can see your total available hours. Also, don't forget to click 'Save Changes'.
Below your Availability chart, there is the option to add exceptions to your Availability. Here you can use a calendar picker to list specific, unavailable dates such as planned vacations, holidays, etc. You can also adjust these selections and mark your exceptions by the hour just as you can in the General Availability table. To remove selections, use the trash icons to the left of each row and, of course, make sure you click 'Save Changes' before leaving the page.