It won't take too long to gather the supplies if you already have an Advent Calendar, so go check it out and get started a few days late if you have to!
I bought my advent calendar with boxes from Hobby Lobby for less than $10 and decorated it using Skip To My Lou's free printable numbers (with help from Photoshop).
Do you want to make your own Advent Calendar? Check out Tip Junkie's DIY Advent Calendar Roundup or search OnePrettyThing for tons of great ideas! Here's one way to make it from Adventures of a DIY Mom!