Start in the Schedules pane in VSP and create a schedule for the time frame of your choosing (must be at least 1 month).  When first created, the schedule will be empty, unless you have any volunteers on repeating assignments.  If there are any volunteers you would like to manually assign to a position, this can be done here.

When you are ready to post the schedule online so volunteers can sign up, go to the File menu and select "Save As...".  Check off "Make this schedule live now so volunteers can see their assignments online", and select "Customize the online layout ("Live Schedule Post") now".  Once you click OK in the Save dialog, you will be directed to specify the schedule settings.

NOTE:  In the Layout Settings pane of the "Online layout and options" dialog, ensure that the "Show unfilled positions in list of volunteers" option is selected so volunteers can see and sign up for the unfilled positions.

To list the unfilled positions so volunteers will see them when looking at the My Schedule tab, go to the Web Terminal pane and click on the My Schedule tab inside VSP.  You can add the "UnfilledPositions" token by clicking on the Token menu, mousing over "Available Positions" in the drop-down menu, and selecting "UnfilledPositions". It is recommended that you add this token to the "My Schedule" section. This way, when a volunteer logs in and goes to their "My Schedule" tab, they will see their personal scheduled dates (should any exist) and also a list of all of the unfilled positions for which they can volunteer.

This will create a list of open positions for each volunteer in the My Schedule tab of the Web Terminal.

Additionally, because you posted the schedule and included all unfilled positions, volunteers can sign up for positions online via the full schedule.  When volunteers view the full schedule via the Web Terminal, all of the empty positions will be listed so the volunteer can select which shifts they want from the calendar view and volunteer.