Do one wall at a time. Do the walls first, and save the windows, doors and trim for after the walls dry. If you're doing the ceiling, always do that first since paint will splatter onto the walls. Cut-in with a brush all the areas that a roller can't reach. Only cut-in one wall, stop and roll it before going to the next wall.
Most people skimp on paint, trying to save money.
When rolling, go slow near the ceiling and trim. Apply slight pressure to the right on the roller handle so the edge you left is cleaned up on each pass. When you're done with a wall, stop and carefully examine your work. With a nearly empty roller go over and touch up any uneven spots while the paint is still wet. Don't touch up later or you may make it even worse.