Ask an owner what staff turnover costs them and the usual answer is a shrug. Put the number together — the notice period worked badly, the weeks of a manager's time spent hiring, the slower service and the mistakes while somebody new learns the floor — and it is routinely more than the salary of the person who left.
Almost all of it is decided in two places. The first is hiring: interviewing for attitude and availability rather than a CV, and being honest about the hours before someone joins rather than after. The second is the first week, which is when a new joiner decides whether this is a job worth keeping. A restaurant that trains properly in week one rarely has a retention problem in month six.
The guides below cover hiring, that first week of training, and the rostering decisions that quietly drive people out. None of it requires an HR function — just doing the same few things consistently.