How can I effectively apply a new coat of paint over existing paint on a wall?

To effectively apply a new coat of paint over existing paint on a wall, follow these steps:

1. Clean the wall surface to remove dirt and grease.
2. Sand the existing paint lightly to create a smooth surface.
3. Apply a primer to help the new paint adhere better.
4. Use a high-quality paint and apply it evenly with a roller or brush.
5. Allow the paint to dry completely before applying a second coat if needed.