Brush: Stiff Bristle Brushes, round and square both work for various application areas.
Product: Works well with most popular brands of acrylic paints, better if the paint is not too thin and if it doesn't get rubbery and ball up in the brush as it dries, also better if the paint does not have a glossy element
(I didn't care for The old brand Studio Stains to Dry brush with due to this.)
Supplies: Absorbent material like scratch paper, (Nothing with ink print that can be picked up in the brush, newspaper is notorious for this) Towel, tile or paper plate (Water - used only for clean up)
Piece: Especially suited for pieces with lots of texture, but can also be used on smooth areas to create a soft look when applied evenly. The basecoat should be darker than the color being used to Dry Brush over the top.
Prep: Base coat the area with a color that is darker than the desired outcome. Black is a typical color used as a base, or other dark colors like Doc Holliday's Real Brown, Steel Grey, Midnight Blue, Shadow green, etc. You can also basecoat a light or medium shade and then antique with a darker shade to achieve the depth in the cracks.
Method: Dip the end of a clean dry brush into paint that is lighter than the base coat and then pounce most of the color off on the scratch paper until the brush marks on the paper resemble "chicken scratch", very rough, very dry, no wet blobs. Swipe the brush back and forth against a textured area getting an all over fuzzy appearance keeping it even and consistent. Repeat this over an area several times, building layer upon layer until you achieve the coverage you desire. Maneuver the brush across the grain in a manner that keeps the bristles out of the crevices, allowing the darker base to stay dark. You can keep adding different color to your brush blending and highlighting. If you want to change to a clean color without this blending affect, you need to use a new dry brush, or clean your brush and completely dry it, working it against a towel until it is dry. This technique does not work with any added moisture in your brush.