I know this is going to seem really easy, but this is one of THE most asked questions about my layouts lately.
First, gather a cup, a basic paintbrush, some craft paint like this from Making Memories, or any other acrylic paint like Claudine Hellmuth's Studio line. Also, have some water on hand.
Next, add just a little paint to your cup. Once you have the amount of paint you want, depending on your project, add just a little water to thin it out. Always add water to your paint, because if you begin with too much water, you'll waste a lot of paint trying to get it to the right consistency.
Mix up the paint and water, then go to town. You can also use a toothbrush for this, and bend the bristles back to splatter paint in all different directions.
Once you're done splattering, and the paint has dried finish decorating your piece with mists and stamps. I used Sunflower Yellow, Orange, and the awesome vintage airplane stamp from the Studio Calico shop.
Then you're done! Add this technique to gifts, art journals and layouts :)