Originally I made these with chipboard coasters. But now I use the heavy white sheet that comes in the DSP packs for the covers; just cut them to 4 inch squares and cover front and back with DSP. Then I use card stock in colors that coordinate with the DSP for the twelve months. I make my own calendar using MS Word each year.
This year I used the retired Every Little Bit stamp set for each stamped image each month. Snowflake, heart, shamrock, umbrella, flower, sunshine, star, bee, school bus, bat, pie and Christmas stocking are all in that set. There are lots of other little images in the set. It's a set I have used a lot!