To get started, you’ll need to create a Photoshop document that’s 25cm wide, 31cm tall and 300dpi.
Create a Gradient fill (Layer > New Fill Layer > Gradient). Click on the default gradient to edit it – change both opacity stops to 100%, then use #dfe8d7 for the left-hand colour stop and #68acbd for the right-hand one to create a sky-style background. Set the Location for the right-hand stop to 70%. Click OK and make sure the gradient is running light-to-dark from bottom to top.
Next, apply a Noise filter (Filter > Noise > Add Noise) with an Amount of 4%, and Uniform and Monochromatic selected. This will make the background less digitally clean.