Sewing a box seam on your doll creates a flat bottom so that it sits well. After sewing a box seam, you could stuff the bottom with heavy items such as sand, poly pellets, or rags to give the doll extra support for sitting on it's own.
1. Pin side seams to bottom seams.

2. Sew a line across, about 3/4 to 1" in from point.

3. Cut off ends 1/4" away from stitching.

~Primraggs