Preparation of the Seder items should ideally be done on Friday.
If the Zeroah or Beitzah (shank bone and egg for the Seder plate) was not prepared on Friday, they may be roasted on Motzaei Shabbos right before the Seder. If this was done, the Zeroah must be eaten on Sunday (as it should not be eaten on the night of the Seder. See Shulchan Aruch Siman 476:1) and the egg should be eaten either at the Seder or on Sunday during the day (and not saved for the second Seder).
If Charoses was not prepared on Friday, it may be prepared before the Seder on Motzaei Shabbos, but it should be grated with a Shinui. Also, only the amount necessary for the Seder can be prepared, and not a larger batch.
If the horseradish was not grated on Friday, it may be grated on Motzaei Shabbos before the Seder, with a shinui.
The romaine lettuce should ideally be checked on Friday, as properly cleaning and checking it on Yom Tov is problematic. If a bug is found on Yom Tov, it may only be removed together with a piece of the lettuce.
The romaine lettuce should not be left to soak over the entire Shabbos, as if it was soaking for 24 hours or more it is deemed Kavush, pickled, and is no longer valid for use as Maror. (See Mishna Berura Siman 473 S”K 38)
Before women start preparing for the Seder on Motzaei Shabbos, they should first say ‘Baruch Hamavdil Bein Kodesh L’Kodesh’. (See Mishna Berura Siman 299 S”K 36)
