Wet the outside edges of the two body parts with a finger tip, then seal with coils of moist clay. This helps the clay coils to adhere better. Leave no gaps; there must be no air leaks.

Check to see that the sides of the voicing opening are straight and level with each other. If not, lay the flat "blade" of tool #10 (or a slot stick) across the voicing opening, and press up from underneath.

Lay the mouthpiece (with slot stick inside) into the opening cut for it. The slot stick should be centered in the voicing opening.

Check the fit around the mouthpiece. If the opening is too large (Figure 3), add coil(s) of clay to fill any large gaps (Figure 4).

Hold the mouthpiece in place with a coil of clay and look down inside it (the mouthpiece) toward the narrow exit slot. You should see the beveled edge (or point) of the lip. (Figure 5 and diagram A on the right) It will be defined by light showing in the top of the exit slot, and darkness on the bottom.
(Figure 6 and diagram B on the right) The mouthpiece is too high; the exit slot is above the point of the lip so no shadow line is seen.
(Figure 7 and diagram C on the right) show when the mouthpiece is tilted too low; all that is visible is the shadow below the lip.

Diagram showing mouthpiece-to-voicing lip alignment.