A method of retrofitting flow-altering devices to an outer surface of a wind turbine blade is disclosed. The flow-guiding devices are of the type having a base comprising an inner side for attaching onto the surface of the wind turbine blade, and an outer side with protruding flow-altering device parts. The method comprises the steps of: a) inserting the protruding flow-altering device parts into a mounting plate so that the inner side of the flow-guiding devices are exposed from a first side of the mounting plate, b) adhering the inner side of the flow-altering devices to the surface of the wind turbine blade by applying the first side of the mounting plate onto an area of application on the surface of the wind turbine blade, and c) removing the mounting plate from area of application on the surface of the wind turbine blade.