Small cards placed at each seat naming the simanim on the table and what is said over them, so everyone can follow rather than waiting to be told. Making them by hand turns a printable into something that goes on the table year after year.
The simanim your family actually uses, in the order you do them. Which foods are included and what is said over each varies between families and communities — follow your own minhag, and ask your rav if you are unsure. The craft here is the card; the text is yours.
An afternoon, plus drying time for the glue and the sealer. The pressed flowers and gold leafing pen are the slow part and the part worth not rushing. Doing all the cards in one sitting is faster than doing them a few at a time.
Sealed with mod podge, yes — that is what the sealer in the materials list is for. Honey and wine land on the table on Rosh Hashanah, and an unsealed card with pressed flowers will not come through it. Sealed and stored flat, they come out again next year.