What sort of preparations was Hashem referring to?
Rashbam: To bake or to cook, as they saw fit. 1
Seforno: To prepare on Erev Shabbos nice foods to enjoy on Shabbos.
Targum Yonasan: He was referring a. to preparing the food that it should be ready to eat (to avoid the Isur of Muktzah), and b. the various types of Eruv that they needed to arrange on Erev Shabbos to enable them to carry from one house and from one Chazter to another. 2
What are the ramifications of the statement "Vehayah Mishneh ... "?
Rashi: When the people arrived home on Friday with the Manna that they had collected, they would discover that the Omer that they had collected had doubled and had become two Omers.
Rashbam: When they went to collect the Manna from outside the Camp, they discovered two Omers for each person in the family, instead of just one, like on every other day.
Seforno: Even after they cooked the Manna, what they cooked for Shabbos had not diminished (as usually happens when one cooks food), but the full amount was still intact.