To add a new shift to your schedule, click on the "+ New shift" button in the upper right corner of the schedule:
Here, you can set the date, start time, and end time for your shift. (Note that you can manually type in start and end times for your shifts, allowing for more precise start and end times than you will see in the drop-down menus.) You can also specify whether the shift is repeating, what kind of pattern it repeats in, whether the shift takes place only between certain dates, and optionally specify a location for the shift as well. Below this, you will also specify how many volunteer spots you need for each job that is used in this schedule.
If the job that you need is not in the list of jobs (or if no jobs have been created yet), you can use the "+ Add another job" link to create a new job.