Buy Christmas stickers (at least 24), a white poster board, and a box of plain white small envelopes from the dollar store. Gather glue (I used a glue gun) and coloring/drawing supplies.
Put a sticker inside each envelope under the flap, and glue all 24 onto poster board. I was able to fit all the envelopes by making three columns and eight rows. Put a drop of hot glue on each corner
Another photo of envelope + sticker
Number each envelope and if you want, create a special design on each one. I combined Christmas designs with drawings special to the person I made this for, like Mickey, Batman, and inside jokes.
Put a small gift, candy or card with a special message in each envelope, and prepare for a huge smile when your loved one sees their own custom advent calendar! Merry Christmas!