A method of assembling a riblet panel (200, 300a-d, 400a-d, 500, 600a-c, 700a-d, 800) for attaching riblets (202, 302a-d, 402a-d, 502, 602a-d, 702a-d) to a surface over which a fluid flows, the method comprising: preparing a plurality of riblet films, each riblet film comprising one or more riblets (202, 302a-d, 402a-d, 502, 602a-d, 702a-d); and attaching the plurality of riblet films to a common laminate film in a predetermined arrangement, wherein the riblet panel (200, 300a-d, 400a-d, 500, 600a-c, 700a-d, 800) is configured to adhere to the surface; and wherein the predetermined arrangement is configured such that, when the riblet panel (200, 300a-d, 400a-d, 500, 600a-c, 700a-d, 800) is adhered to the surface, the one or more riblets (202, 302a-d, 402a-d, 502, 602a-d, 702a-d) of the plurality of riblet films are configured to reduce the
skin friction of the surface when the fluid flows over them.