How do I set up a volunteer to be scheduled exactly one time every week?

Any fixed, repeating volunteer patterns, such as someone  serving at the same service in the same role every week, should be scheduled using preassignments. 

1. Click on the Preassignments pane in an MSP installation or in the Admin view of the Web Terminal. 
2. Click the "New Preassignment..." button in the upper right corner.
3. Select your preferred volunteer and specify the pattern for which you’d like them scheduled.

(Setting a volunteer's preferred serving frequency to "one time per week" will not suffice. That setting is interpreted as a preference, and does not guarantee that the volunteer will be scheduled exactly one time per week.)