Brush a little water around the edge of the circle, fold the dough over the filling and seal the seam with your fingers. (lilmisscakes.com)
Use your finger to wet and seal the seam on the outside. (emilybites.com)
The key is to add the last flour to the dough slowly, creating a firm dough, one where the dough will seal a seam only with some effort. (sourdoughhome.com)