Measure and cut a back seat support for the chair seat by determining how much you want the sides of the chair to overhang the seat supports, and cutting the back seat support to a length which is shorter than that by the width of both back legs and the width of twice the amount of overhang you want for each side. Glue the back seat support to the back of one side of the chair, so that it will be at least the thickness of the chair seat higher than the side supports. The back of the chair seat is butted up to this support and glued, it does not sit on top of this back support piece. For a more detailed chair in future, you could cut out the back corners of the chair seat so it fitted over the back support brace.
Measure and cut the front seat support the same way. For my 1:12 scale kitchen chair I needed a back seat support that was 13/16 of an inch long. The front seat support was 15/16 of an inch long.
Glue the front seat support square with the front corner of the chair side as shown in the photo above.
Leave the chair sides to dry completely.