It's pretty basic really. I use a small piece of household sponge, some gouache (pans) and these:
That's right, stencils stencils and some more stencils. I love me some stencils! These are all 15x15 cm (6x6 inches) or a little bigger, which is the most practical to work with inside a book. I basically either keep the page white or fill it with a layer of colour (again using the sponge) and then I pick a stencil and a matching colour and sponge the patterns on the background.
That can give results like these:
Hope this clarifies things. If you want more ideas for using a sponge and watercolour, I have an old tutorial on it in the tutorial section, or you can just click here.
Wishing you all a wonderful and artsy week!