Step 1)

Make your background image edits in your graphic editing software and save out. If you need transparent areas around your image, save as a PNG. If you need to save out a standard image, save as a JPG.

When we design the User Interface for your apps, we typically create the whole background with all graphics and text and save out each as individual layers so it can move and adjust depending on the screen size.

Think of the size of your buttons as a ratio of 100%. How much of 100% of the width of the screen or height of the screen should they take up?