Sand the entire surface of the drywall, using a sanding head attached to a painting extension pole. Rub your hand over the drywall to check for roughness. Sanding the drywall removes rough texture that can cause streaks to appear underneath dry paint.
Roll the primer onto the entire wall, using a 1/2-inch paint roller screwed onto the end of a painting extension pole. Prime severely abused and stained walls with an oil-based primer; otherwise, use a latex primer. Primer bars pesky stains from bleeding through the paint, preventing streaks.
Open each separate paint can of the same color and dump the paint from each can into an empty bucket. Dump the desired amount of paint into an empty 5-gallon bucket. Stir the paint meticulously, using a long wooden stir stick. Stirring paint ensures the pigment is mixed evenly and stops subtle color streaks in the finish.
Cut in around molding and ceiling edges, using a clean paint brush saturated in a quality paint. Brush paint lines approximately 2 inches wide, so the paint roller can reach the edge without bumping the ceiling.
Screw a clean roller, preferably an unused roller, onto the painting extension pole. If you use an old paint roller, wash off old paint residue with warm water and use a metal roller spinner to remove the water. A watery roller, or a roller with traces of old pigment, can leave unsightly streaks on a wall after rolling.
Dip the clean paint roller into the bucket and begin rolling the wall, working only in one direction. Spread the paint evenly on the wall until the roller looks dry and the paint appears thin. Return to the original starting point and quickly re-roll the paint in the same direction to reduce stipple. Allow the paint to dry and apply a second coat.