In about 3 evenings, I cut, painted, and installed all the window trim and window plastic. It's made a huge difference in the appearance of the house! I have a little more touch up to do where the joints are, but you get the idea.
The downstairs arched windows
One of the bedroom's round windows
The "library" or reading nook
Also, I made a fabric seat cushion for the box window seat. This one was pretty easy since it's rectangular, but this will also work with an angled bay window seat. Here's a super simple tutorial on how I did it.
No-Sew Window Seat Tutorial
Take a piece of thick cardboard trimmed about 1/8" smaller than the opening. Cut a piece of fabric about 3/4" bigger than the cardboard. It doesn't have to be perfect. I started with the narrow sides. Place a tiny line of tacky glue on the cardboard, fold the fabric over, and use some pins to hold the fabric in place. Make sure it's pulled smooth.
The corners were folded in at an angle so it made a nice edge on the front. I used a tiny dab of glue to hold the fabric at that angle. If you use too much glue, it will seep through the fabric. Apply another tiny line of glue along the backing. Fold the fabric up and pull it taut, not too taut so that it distorts the pattern, just enough to make it smooth.
Pin it in place. Remove the pins when dry.
Slide the cushion in place in the window seat to make sure it fits. Remove and apply some tacky glue to the seat underside and replace. I used a small can to hold it down until the glue set. If your window seat has thick lumps of glue and fabric, trim those down with a craft knife or small scissors before gluing. Lumps will make your seat look funny.
To complete the box window, I'm going to use trim around the opening, perhaps frame trim, perhaps somethings else. You'll just have to wait and see.
Now, I'm on the hunt for some lace to use as curtains. I already looked in my supply cabinet and found nothing good. Also, I need to order some more baseboard trim since I don't have enough to do the whole house.