For the arch windows in my house, I used a curtain rod from Home Depot that is flexible. You put the curtain on the rod and then bend it into the hooks inside the arched window. You can get fancy and create your own curtain (I did, one white side one colored side) or you can just purchase a regular curtain. Once installed, any excess curtain can be bunched up and tied with a rubber band or it can hang loose.
For my daughter's window, I bought some extra material that matcher her bedding, sound proofing material, and blackout material and sewed it all together. The curtain is very thick and does block out a lot of the cold.
So those are the fancy options. Now the quicker and really not fancy ideas: For the arched window...by the plain accordian paper at Home Depot that you can cut to length for the window. It's like $5 per window. That's for the outside appearance. Then stuff throw pillows on the inside for insulation. They can also be removed during the day to let the sun/heat in. For the bigger windows, cover them with old thick heavy blankets. You can drape them over a curtain rod, or if you don't care about the walls, you can thumb tack them in place temporarily.