I came across this idea in a book called "Watercolor Wisdom" by Jo Taylor. (See the book here.) It's a really fun way to start off a painting.
First, you might want to have some idea of where your light, medium and dark values will go... then squeeze and drag the pure gooey pigment along some of the edges... (or you could use a palette knife)
Next, load a big brush with just water, and stroke the drippy brush over the wet paint, mixing the colors on the paper. Keep adding clean water (rinse your brush) to the moist paint on the paper, pushing and mixing the colors in the direction you want them to go.
I added some scraping too, in the example below. It's not quite finished... but is a good start to finish up later!
Next time you are scratching your head trying to decide what to paint... grab a couple of tubes of paint and draw with them directly onto the paper... Then all you need add is water!