In order to make a personalised helmet liner using a rigid injection shell, a cast of the user's skull is made from a quick-setting product; the desired position of the user's head and the cast relative to a predetermined helmet reference frame is determined on an optical bench and a reference support (30) is fixed to the cast; a dummy head provided with a positioning support relative to the rigid shell is moulded in the cast by comparison with the reference support; the dummy head is placed inside the rigid shell and the dummy head and the shell are positioned relative to one another by using an indicator (31) on the support, and an expandable foam is injected into the space between the rigid shell and the dummy head.