Start by clicking on the Gradient tool in the Tools palette. Then up at the top of the screen in the Options menu, you can change the basic shape of the gradient.
Next, click on the Edit button and the Gradient Editor window will open.
To take this to the next level, add a ScrapSimple paper template, and apply your gradient to it by using a clipping mask. I love this wavy paper template from June’s ScrapSimple Club: Beyond the Sea by Laura Louie. A gradient is perfect for this template because it accentuates the movement of the waves.