A method for forming a piston of an internal combustion engine, said piston having a piston head and a piston skirt which extends from said piston head, wherein the piston head has a lateral surface in which circumferential slots (5, 6, 7) are formed for housing the respective piston rings, wherein the piston skirt has a portion of reduced diameter adjacent to said piston head, so that the circumferential slot which is farthest away from the top surface of piston is delimited, on its side facing towards the skirt, by a circumferential rib projecting radially outwardly with respect to the wall of said adjacent portion of the skirt, wherein in said circumferential rib there are formed, starting from its outer peripheral edge, one or more through notches, which extend throughout the entire thickness of said rib and are configured for defining drainage passages for the engine lubrication fluid, through which, in the operative condition of the piston, said circumferential slot which is farthest away from the top surface of the piston communicates with the annular space defined between the wall of the engine cylinder and said portion of reduced diameter of the piston skirt, wherein said notches are formed by means of a material removing operation in a stage subsequent to a first forming step for forming the piston body, said method being characterised in that said notches are formed by means of a machining operation, with the aid of a rotating milling tool which is displaced radially with respect to the piston axis, while the axis of the milling tool is maintained parallel to the piston axis or slightly inclined with respect to a direction parallel to the piston axis. said piston having a piston head and a piston skirt which extends from said piston head, wherein the piston head has a lateral surface in which circumferential slots (5, 6, 7) are formed for housing the respective piston rings, wherein the piston skirt has a portion of reduced diameter adjacent to said piston head, so that the circumferential slot which is farthest away from the top surface of piston is delimited, on its side facing towards the skirt, by a circumferential rib projecting radially outwardly with respect to the wall of said adjacent portion of the skirt, wherein in said circumferential rib there are formed, starting from its outer peripheral edge, one or more through notches, which extend throughout the entire thickness of said rib and are configured for defining drainage passages for the engine lubrication fluid, through which, in the operative condition of the piston, said circumferential slot which is farthest away from the top surface of the piston communicates with the annular space defined between the wall of the engine cylinder and said portion of reduced diameter of the piston skirt, wherein said notches are formed by means of a material removing operation in a stage subsequent to a first forming step for forming the piston body, said method being characterised in that said notches are formed by means of a machining operation, with the aid of a rotating milling tool which is displaced radially with respect to the piston axis, while the axis of the milling tool is maintained parallel to the piston axis or slightly inclined with respect to a direction parallel to the piston axis.