There are lots of ways, with sheets, drop cloths, fabric. I've seen oodles of great ideas on Hometalk -- did you check around using the search bar at the top left? I'm sure you'll find a bazillion ideas that way.
They are rather simple, a closed hem down each side about 1/2". Then a 4" hem at the bottom. Decide what you want at the top. Easiest way would be to add a hem at the top with a pocket if you want a ruffle at the top.\
You can probably buy them cheaper than making them. Check jcp.com.
Are you looking for sewing ideas, or non-sewing ideas?
here is how I made no sew blackout curtains https://www.charlestoncrafted.com/no-sew-fully-lined-grommeted-window-curtain-panels/
If you decide to adjust exisitng curtains I find the easiest way is to take off the header tape and fold down from the top to make shorter and then reapply the header tape (I have just made one pair of very large curtains into two pairs for my parents and kept the exisiting hem, much quicker!)