After the Shabbat morning meal(s) are eaten and the remnants of the chametz are discarded but not later than the fifth halachic hour of the day, the chametz is again nullified by the recital of Kol Chamirah.
Seder preparations should be completed before Shabbat. These include roasting the shankbone and egg, mixing the salt water, making the charoset, grating the horseradish and checking the lettuce for insects (Rav Dovid Feinstein). In view of the fact that one may not prepare on Shabbat for Yom Tov, the Seder table should be set after Shabbat ends.
Because of differing opinions in many of these matters, please consult your rabbi.