Widen the gash slightly with a sharp, hook-type opener, such as an old bottle opener. Dig out any broken drywall by inserting a screwdriver or stick into the gash. Tear off or push in the wall's paper covering to avoid a crease in the patched surface.
Cut a piece of mesh tape large enough to cover the gash. Lay it over the gash on the wall.
Lay a rough coat of joint compound over the tape. Feather out the edges to create as smooth a finish as possible. Let dry.
Knock down any high spots of dry joint compound with a utility knife. Apply a second coat of joint compound, feather the edges and let dry. Remove any rough edges or high points.
Apply a wider and thinner coat of joint compound as the final coat, and let dry. Wide or deep cracks typically require two or more applications, and the final coat should be less obvious for best paint results.
Lightly sand the wall with fine-grit sandpaper. Wipe the dust off the wall with a tack cloth.
Apply an interior primer, then paint the wall after the primer coat is dry.