To sit in the seiza style, one first kneels on the floor and rests the buttocks on the heels. This position is called kiza. In some cases, kiza is considered a type of seiza in the world of martial arts or tea ceremony.
To sit on a zabuton in a proper way, you are supposed to sit in the kiza style at the back of the zabuton and move to the center of it to sit on by izari (see the description for izari). This is because stomping on a zabuton is considered an extremely bad manner.