Automatic mode matches features against the vanilla 16x16 texture, then verifies every face by pixel correlation. It can fail on hard shots. Then draw one tile quad (click its 4 corners, then drag to adjust) and grow from it.
Click Draw tile quad, then the 4 corners of one grass-top face clockwise from top-left. Enter its grid x, height y and z, then Add face; rotation is classified from the pixels. Grow tries to find neighbours from it. Detected rows stay editable in the table.