You make it as one large piece that you press in a wooden mould to give it a uniform rectangular shape, then you cut it up into individual pieces.
You soak the mould first in water for 10 minutes, then brush the insides with tezu.
You build the piece of sushi in reverse. You put the top ingredients in the bottom of the mould, then the sushi rice, then a piece of wood to press down with, then press, and put a weight on top, and leave for 10 to 15 minutes.
Then you unmould it, and slice into pieces.
Sometimes it is sprinkled with seasoning, which is unusual for sushi.
The layers will vary be recipe of course. Typically, for the bottom or top layers sushi rice, nori, smoked salmon, etc, are used, with other ingredients in between.
Language Notes
Aka “hakozushi” (“hako” meaning “box”), aka Kansai style.