Ignore the dry erase board underneath--I left a note for my roommate. I started by making all the numbers and letters because that was the biggest job. You need numbers 1-31 and enough letters to spell out each month. I took a sheet of paper and listed all the letters in the alphabet down the side. Then I went through each month and tallied up how many of each letter I would need. You can use anything that you can stick a magnet to. I used a lot of scrapbooking stuff, but also used a quarter (for 25), two dice (for 15--one dice has the one dot facing up and the other five), and a ruler (for 12). Make sure you stick a magnet on the back. I preferred the magnets that were self-adhesive because they weren't as messy. I believe the magnets came from Walmart and Joann's.
Here you can see closeups of some of my numbers. I did the same thing with the letters.
The next step will be to figure out how you are going to hang it. I had my dad drill two holes in the top lip of the cookie sheet and tied it with ribbon. Next I cut a piece of scrapbook paper to fit and divided it into six rows. I used a sharpie to draw my lines. The top row is for the days of the week--I didn't use magnets on these because they don't change. I glued the paper and the days on the cookie sheet using a spray adhesive. I would suggest sealing it with some sort of clear coat because mine was a little sticky at first because of the spray adhesive. That's pretty much it. Mine hangs in my kitchen. I like the tray-type cookie sheets (I don't know what the technical name is) because I put all the pieces I don't need for that month on the back to store them.
You can also make magnets for special events. I have cupcakes for birthdays and a heart for Valentine's day, and the word "vacation." I also did a cow, chicken and pig to represent a barbecue contest I volunteer for. Pretty much the sky is the limit.
I did another variation for a friend that was a little more matchy. For the numbers, I used a punch and cut tags out of paint samples then used scrapbook stickers to put the numbers on them. I also make the months match as well. It was super cute, but not quite as eclectic as this one.