How to paint a wall with a roller effectively?

To paint a wall with a roller effectively, start by preparing the surface by cleaning and sanding it. Then, apply a primer if needed. Use a roller with a nap that suits the texture of the wall. Dip the roller in paint, remove excess, and roll in a "W" pattern. Work in small sections, keeping a wet edge to avoid visible lines. Finish with smooth, even strokes for a professional look.