cardstock (in two colors, three if you want a contrast star)
an x-acto knife
a cutting mat (or in my case--the back of my sketchbook)
fun patterned washi tape
freehand cut both the squares (approximately 2" x 2") and numbers. tape them up to the wall into any shape you like. i did a tree, but it could be fun all in a row or as a square too. as the days go by, tape a contrast color to the back of the numbers until you reach the 24th.
i love that against our white wall it looks very much scandinavian! :)