In order to install the windows and doors, you must paint each of them while still on the sheet. It makes it easier to do so. After they are pinched out of the sheet, you just need to touch up the edges with paint.
You will have to use tweezers in order to handle and install these small parts.
The window and door trim, does not have to be painted on both sides because they are glued flat on the wall, at their pertaining openings.
The doors and windows do have to be painted or stained on both sides because they fit inside of their pertaining openings, so their backsides will be visible from the interior of the dollhouse.
I painted the trim but stained the windows and doors.
After they are in place, you will have to do touch ups and it has to be a very meticulous process. Use very fine tip brushes.