The invention relates to a method for placing a flexible jacket 3 on a natural vessel section 5, in particular a vein, to a device for placing a flexible jacket 3 in accordance with said method, and to an auxiliary instrument 1 for use in said method. The method comprises the following steps: pushing the jacket 3 onto a tubular application aid 2, filling the vessel section with a fluid, introducing, in particular pulling, the filled vessel section 5 into the application aid 2, the filling of the vessel section and the introduction of the vessel section 5 into the application aid 2 being carried out with the assistance of a tubular auxiliary instrument 1, and pulling the application aid 2 and the tubular auxiliary instrument 1 apart while at the same time pulling the vessel section 5 out of the application aid 2 and depositing the jacket 3 on the surface of the vessel section 5.