Once the sink support shelf is glued in place and the glue has dried, carefully turn the sink cabinet back right side up and remove the farmhouse sink. The upper hinge pins can now be fitted through the sink support shelf. Test fit the hinges to make sure the doors will open smoothly without bashing into one another.
If you are putting a regular dolls house sink into your cabinet instead of a farmhouse or butler's sink, cut out the hole for the sink in the top of your cabinet using a template, and glue the cabinet top in place over the sides. Cut a drawer front to fit the space in front of the sink and glue it in place between the top of the sink and the sink support shelf.