Here's the cover and first four months for this year's "Coaster" calendar. Instead of coasters I use the white tag board backing the comes in the packs of Designer Series Paper. Yes, I save those, too. I have a small space on one of my studio shelves where I store them.
I use that tag board cut to 4"x4" to make the cover and back of the calendar. The month pages are just a piece of 4"x4" card stock.
When I'm making these calendars I pick a pack of DSP that has at least 6 different colors in it and then pull card stock in those six colors. Each monthly page has a small piece (1.5"x4") of DSP glued to the bottom of the cardstock. I usually think about what ribbon I have to trim the pages, too. This calendar is made using the retired Sycamore Street DSP. Then I decide what little stamp, punch or embellishment I can use that goes with that month.
Once you have those things decided the work goes pretty quickly. Since I was making a couple of these for the church's Holiday Fair I stamped and cut everything multiple times and then got busy putting the pages together.
I used my Crop-a-Dile to cut the holes for the rings.