Reading, singing,and playing. Teaching to be nice to one another and sharing. Also being emotionally connected to the children and caring. Communicating to the parents of any misbehaving or new behaviors, good or bad.
There is an actual curriculum that, when applied correctly, does develop all of the skills needed for a child to be ready for kindergarten. As with all things, end quality relies a lot on the people executing the plan - but the plan is solid and that is the first piece necessary to determine what staff, facilities, and processes, should look like.