Sand the surface of the satin-finished wall using an electric sander with a medium-grit sanding belt. This will level out any uneven areas of the wall and will smooth down thicker paint.
Brush off the wall using an 18-inch spackle brush to remove any debris, then vacuum the area with a shop vacuum.
Fill a paint tray with matte-finish latex wall paint, any color that you prefer.
Coat the end of a 3-inch flat paintbrush with the matte paint, then apply the paint to the top and bottom edges and corners of the wall.
Coat a paint roller in the matte paint, then apply the paint in an even layer over the entire wall. Allow the paint to dry for 12 to 18 hours.
Apply second and third coats of matte paint as desired, using the same method as with the first coat. Allow each coat to dry for 12 to 18 hours in between each application.