How do I paint a room without streaks or lines?

To avoid streaks and lines, start by using a quality roller with the right nap thickness for your wall texture, typically 3/8 inch for smooth walls. Apply paint in a W or M pattern, then fill it in without lifting the roller, keeping a wet edge at all times so you're always blending into fresh paint rather than dried paint. Use a good brush to cut in along edges first, then roll while that paint is still wet so the two blend together. Apply two thin coats rather than one thick coat, and let the first coat dry completely before adding the second. Avoid pressing too hard on the roller, which causes streaking, and maintain consistent pressure throughout.