buy one of the cleaning kits for the particular brand your filter is, it generally involves gently tapping out the bigger particles trapped in the filter, then applying a degreasing agent spray that cleans away the oil and washes out the dirt, then a gentle rinse (from the top side that is closest to the engine to the bottom side, closest to the air feed from the bumper to stop forcing the dirt further into the filter) let it naturally dry then completely re-oil and insert back in the car.