For the cookie sheet background: Be sure the cookie sheet is clean and dry. Prime the cookie sheet using spray primer.
Paint cookie sheet using acrylic paint.
Select a 12x12-inch sheet of card stock for the calendar grid. Trim the card stock width to fit within the edges of the cookie sheet. Draw a calendar grid in pencil and then use a marker to go over the grid.
Arrange patterned papers and calendar as shown, trimming as needed to fit the cookie sheet. Use decoupage medium to adhere papers to the cookie sheet; let dry. Add 2 or 3 additional coats of decoupage medium over entire cookie sheet to seal.
After decoupage medium is dry, glue the letters for "Christmas" to the top part of the cookie sheet.
For the day-of-the-month pieces: The 25 pieces for the 25 days until Christmas should be all different and backed with a small piece of magnetic tape so they cling to the cookie sheet. Use pieces of chipboard as a base to create many of the days. Add gel shapes, glitter, additional small pieces of chipboard, and buttons to the main pieces of chipboard to create the individual magnets. See the photo for ideas. Also look for purchased dice or game pieces, premade chipboard shapes, small toys, and small cookie cutters to use as day-of-the-month pieces.