Don't sew on the hair just yet onto the body. Now you have to sew one side of the body, actually only untill the neck.

Once you have done sewing untill the neck, take the hair a place it on the head and then sew. Now you may sew the head. Then you sew untill the other side of the neck. When you're done put in the fluff or sand or... whatever in the doll. Put only the  fluff or whatever in the arm , leg and tummy.

 You can now sew the body untill the leg. OH!! wait! not yet!

Before sewing  you can make a heart for your doll.