Make sure you paint the top vergeboard before installing the dormers. This vergeboard will need slots to be filled with spackle and then sanded down for proper finishing.
You should also paint around the exterior of the dormer opening to match your dormers interior. This edge is not very visible but you want to go ahead and finish it as if it was. Once the dormer is in place, you won't have access to it again. The dormer will not sit perfectly on the edges of the opening, so there will be a slight exposed edge that can be seen through the dormer windows, especially if you made them working windows like mines.
You can then glue your fully assembled dormer, onto the opening. Your main concern at this point is for the dormer to look straight from the exterior of the dollhouse. There is a possibility that "straight" will not match the opening exactly and that's okay. Just don't end up with crooked dormers.
I recommend you glue the bottom and tops of the dormers onto the mansard panel. Do not worry about the sides. There is a possibility that there will be gaps on the sides because the mansard panel curve will not match the dormer side wall curve. That's find. Those gpas will be hidden later with shingles. Just fully concentrate on the dormers looking straight. When they look perfectly straight from the front and side views, hold them in place with masking tape until the glue dries.
I also went ahead and added the vergeboard trim as well and used spackle on the corners.
