You will also need a fabric that will take the dye. I've done this technique with cross stitch fabric (which is what was used for this instructable) as well as 100% cotton and 100% silk scarves (my girl scouts loved that one!).
The last things you will need are an iron, a spray bottle, and something to put underneath the fabric (I usually use a trash bag).
I made this one at TechShop Austin (awesome place - I'd recommend going out and taking a tour if you have a chance). www.techshop.ws
Step 1: Spray the Fabric
This will help your fabric to lay flat (especially if it's cross stitch fabric that has been rolled up).
Step 2: Lay Out Tissue Paper
As expected, colors will bleed together. If you overlap blue and red, you'll get a purple color in the overlap.
I purposely made strips for the sky so that I had a wispy cloudy look. In the end, it was a little too light, so I added some darker blue (you can see this in Step 3).
Step 3: Spray the Tissue Paper
Step 4: Remove Tissue Paper
Then, remove the strips of tissue paper. If you have a lot of strips overlapping each other, you'll get large sections to come off at once.
Step 5: Iron Dry
Put a towel or paper towels down over your ironing board - the colors will bleed onto your ironing board cover.
Also, the colors will sometimes stick to your iron, so start where the colors are the lightest or else you might accidentally have dark color over your light colors.