In time, committing seppuku came to involve a detailed ritual. This was usually performed in front of spectators if it was a planned seppuku, not one performed on a battlefield. A samurai was bathed, dressed in white robes, and fed his favorite meal. When he was finished, his instrument was placed on his plate. Dressed ceremonially, with his sword placed in front of him and sometimes seated on special cloths, the warrior would prepare for death by writing a death poem.

With his selected attendant (kaishakunin, his second) standing by, he would open his kimono (robe), take up his tantō (knife) or wakizashi (short sword)—which the samurai held by the blade with a portion of cloth wrapped around so that it would not cut his hand and cause him to lose his grip—and plunge it into his abdomen, making a left-to-right cut. The kaishaku would then perform dakikubi, a cut in which the warrior was all but decapitated (akin to a coup de grâce. The maneuver is done such that a slight band of flesh is left attaching the head to the body). Because of the precision necessary for such a maneuver, the second was a skilled swordsman. The principal agreed in advance when the kaishakunin was to make his cut. Usually dakikubi would occur as soon as the dagger was plunged into the abdomen. The process became so highly ritualised that as soon as the samurai reached for his blade the kaishakunin would strike. Eventually even the blade became unnecessary and the samurai could reach for something symbolic like a fan and this would trigger the killing stroke from his second.

— Wikipedia on seppuku