You can access the Availability page on mobile by tapping the calendar icon on the Shifts page.
This page shows a list of your existing patterns. You can edit a pattern by tapping on it, or delete it by tapping-and-holding. You can add a new pattern using the "+" button.
Creating/Editing an Availability Pattern
Enter a Title for the pattern, and set the Effective dates (either "Today Onwards" or "Specific Dates". If you choose "Specific Dates", you'll need to enter a start date, end date, and how often you'd like the pattern to repeat.
Under the form at the top you'll see a list containing the 7 days of the week. Each day has two buttons: Add Availability and Add Unavailability. Using those buttons, you can set periods during each day of the week when you're available/unavailable.
When you're done setting all that up, scroll right down to the bottom and tap "Add".