Turn the clay slab over, and bevel the the opposite edge. (black arrows)

The slab should look like this. The bevels are highlighted in blue and yellow to better define them.
The two edges will be be overlapped - A (blue) over B (yellow). Beveling will reduce the bulk of the seam.

Figure 1 Start at the wide end of the clay slab, and wet the beveled edges. Figure 2 Press the wet clay edges together firmly, side A over side B.
Inset shows close-up of overlap.

Support the seam from inside while pressing it closed. Continue working toward the narrower end.

Finally closed. There will be an opening left at the narrow tip. This will be closed later.
The fold of the clay will become part of the "bottom" of the ocarina.

Figure 3 Use your thumb to finish sealing the seam, and the fettling knife (#4) to smooth it. (Figure 4)