"Score" the cut edges of both pieces with the fettling knife (tool #3). (Figure 1) Apply slip to the scored edges. (Figure 2)

Press slipped edges together, lining up the two marks. Slip will be squeezed out. (Figure 3)This is normal.
Remove the squeezed-out slip with a finger or the knife. (Figure 4)

Use a squared corner of the trimming tool (#1) to carve a channel along the seam line.

Apply slip in the groove (Figure 5), and press a small coil of clay into the "slipped" groove. (Figure 6)