After eating the whole meal, each person eats a kezayit from the shemura matza that was wrapped up. This is done to remember the Korban Pesach which eaten on a full stomach. One should eat the afikomen reclining. Do not make a bracha on eating the afikomen. Be sure to eat the afikomen before halachic midnight. Rema: Try to finish Hallel before halachic midnight as well.
If one forgot and did not eat the afikoman and did not remember until one washed one's hands or one said Let us bless, one eats the afikoman without the blessing of The One Who Brings forth. And if one had not remembered until they blessed the Blessing over the Sustenance, if they remembered prior to blessing Creator of the fruits of the vine, one should wash one's hands and bless The One Who Brings forth bread and then eat the afikoman. And if one had not remembered until after they had blessed Creator of the fruits of the vine, one should not eat the afikoman, and rely upon the matzah one ate amidst the meal, since all of it was guarded from the time of kneading. But, in a place where they are accustomed to do the guarding of the matzahs, it is a miztvah from the time of reaping, even if they hadn't remembered until after Hallel, one should wash one's hands and bless The One Who Brings forth bread, and then eat the afikoman. GLOSS: And [if one had gone back and ate the afikoman after having said the Grace After Meals], one should go back and bless upon the cup [of wine over the Grace After Meals] and one need not worry of adding to the cups. And if the afikoman is lost, one can eat an olive's-worth of another guarded matzah.