I usually put a plastic bag over my head when I dye my hair, but since I dye my hair so often I decided to make something more sustainable that I don't have to throw away after each dye. It's better both for my wallet and the environment.
The main reason why I cover my hair in the first place when I dye it is so that I won't smear color all over the place (I'm very tall and have a tendency to bump into things) but it also keeps the dye wet and it makes it easier to rinse out.
Have in mind that when I created this cap I was in a great hurry so my cap might not be the prettiest in the world, but it's totally functional.
You will need an old rain poncho or some other plastic fabric. Put it over your head (but don't choke!) and decide roughly how much fabric you will need.
Either you cut out a circle by free hand, or you fold the fabric twice and cut it like in the picture.
Either way you should end up with a circle.
Take the elastic band and measure around your head how much you will need. The elastic I used is not the ideal (I would recommend a plain white one) but I got it cheap and it's pink and pretty.
Zigzag the elastic onto the fabric and keep the elastic stretched out so that when you're done it will ruffle nicely. Since my elastic sucked I had to first sew like in the picture, and then I had to take another elastic and repeat the process over the first elastic to make it small enough to fit my head. Silly elastic!
And this is the final outcome! And you can tell from the picture that the elastic is kind of bulky and weird due to the silly elastic and lack of time. But the showercap works great. Don't forget to rinse it off after each use.