This video provides a quick overview of the self-roster settings and how to set one of two types of self-roster:
Selection-based rostering, where users pick shifts to the exclusion of others on a first-selected-first-staffed basis
Preference-based rostering, where users can express preferences for shifts - even if others have already done so - and these preferences are used to build a roster.
There is an additional video here that explains how to build the rosters at the end of a preference-based rostering self-roster process.