Just press space to move between screens. Left and right arrow keys will move at the appropriate times. https://scratch.mit.edu/projects/304642094/ is a very simple example.
Many Scratchers seem to find scrolling difficult to understand but it needn't be difficult. So if you're ever wondered why you're setting up a backdrop sprite to have x position = scrollX + (480 * 3) this might help. https://scratch.mit.edu/projects/100450683/ is a good scrolling tutorial.
