Method for the modular assembly of a pusher to a watch case (100) including a preliminary step of assembling a sealed functional module (10), including a tube (7), a stem (8), at least one sealing gasket (5) and a return spring (3) arranged between a first fixed stop member and a second moveable stop member, and then a first step (E1) of assembling the sealed functional module (10) to the case (100), and finally a second step (E2) of assembling a pusher head (1) to the sealed functional module (10).