To start, you're going to have to decide how many to buy, unless you already have a pile at home. If you already have even one, you can measure it, measure the window, and then do a little math to see how many will be needed to make long enough/wide enough curtains. I did it by eye, holding up one bandana near the top by the curtain rod and gauging approximately how many I'd need.
Once you buy the number you want to use, they'll need to be washed and ironed. I didn't wash mine first, and the end result came out pretty stiff, so I'd highly recommend washing them to soften them a little. If you want a more distressed and/or faded look and you're not itching to assemble them right away, wash them over and over. Definitely iron them to get rid of the creases.
Since the window I was making the curtains for is the smallest window in the house, I decided to cut mine into fours to maximize the number of colors I could use.
Next, you'll need to lay them out to arrange your colors:
When it comes to the sewing part, you can stitch it together however you're comfortable doing it. What I did was stitch together each horizontal pair first:
....and then stitch all the pairs together:
It turned out I miscalculated, and it came out too long. To remedy this, I folded down the top row and hemmed it as if I'd planned all along for the curtain rod channel to look like that. Shhh...don't tell anybody.
After you've created a channel for the rod, make the curtain for the other side the same way. Here's what I usually do to make sure that both curtains are exactly the same length: Lay the second curtain on top of the first, finished curtain, and make sure the bottoms match up perfectly. Then, fold down the top until it lines up with the top of the finished curtain underneath and pin the crap out of it right at the fold, so that when you stitch down the channel for the rod, it won't be able to move and become uneven:
Now you're all set to hang them. Mine are looking a little misshapen and stiff, as I mentioned (wash them first!), but they fit just right:
Here they are pulled back. I used matching ribbon and a thumbtack to pull them back, since we don't usually close the bathroom window curtains. (The glass is frosted.) If you'd like to be able to open and close yours, use scraps from the same bandanas you used for the curtains to make tie-backs, and if you create button holes near the ends, you can attach them to hooks.
Yay new curtains!! They put me in mind of the old Raggedy Ann and Andy books we had as kids.