Here's how I made my curtains. First, I took a navy blue twin sheet. I cut it in half. (Actually, I cut a slit at the top and tore it the rest of the way, which is always fun.) Except for the top few inches (where the deep hem is), I sewed a line of stitching down 1/4 inch from the raw edge. Using this as a guide, I turned under a narrow hem by folding on the line of stitching and folding the raw edge under. (I'm not sure if that makes sense, but I'm having trouble describing it. I guess that's why I don't write pattern instructions.)
Then I cut the closed sides off of the deep hem at the top of the sheet so I could stick the curtain rod through. I didn't bother finishing those raw edges since they're so high up and no one will see them.
The project took longer than I thought it would, about an hour and a half, mostly because the sheets are so long and the seams just took a long time to do. It was easy and cheap (free old sheet, $10 curtain rod), and the curtains do a good job of making the room darker so I can sleep better. They are not very thick curtains, and they let light in during the day, but I don't sleep during the day, so I don't mind that. It's actually a bonus because then I can start to wake up as the sky gets lighter.
I was going to add a valence at the top because I have a really pretty one (it came with a quilt I have). But there were topological obstructions to having the valence up there and still having full range of motion of the curtains without adding a second curtain rod, so I decided the top of the curtains is attractive enough and left it at that.