The key to this chair is to fit the back so that your character figure or doll will fit into the chair. Test that your character will be able to sit on a half shell of a walnut, with the rounded back of the walnut forming the front of the chair seat.
To make the back, you will likely need more than one half of a walnut. Measure your walnuts against one another to find halves which are roughly the same measurement across. Cut the ends from one walnut to make a section without ends, and cut off the pointed end of two other walnut halves, or the pointed end from one half and the rounded end from another half.
Measure your figure from the base of the hips to the top of the head, and make sure your assembled back will fit your figure, with a bit of extra space to allow it to be glued into the seat section (see photo).