Dip the paintbrush in paint. Wipe the brush on the end of the paint can or pan to remove excess paint.
Turn the paint brush sideways in your hand rather than holding it upright.
Paint the crease with the brush turned sideways. When the brush is sideways, the bristles can get inside the crease easier. Move the brush from top to bottom inside the crease until it is completely painted.
Fill in any deep creases with a small artist paintbrush, size 1 or 2. Since the brush is smaller and has smaller bristles, you will have more control over the application of the paint. A smaller brush head can also fit into shallow creases and deep crevices.
Blend the crease paint with the rest of the wall using a 2-inch trim brush, after cutting in or filling your crease with paint. Blend the paint into and near the crease using a series of short strokes. Stroke away from the crease or the edges, rather than toward them, when blending with the trim brush.