Use pliers to remove the staples from the calendar, then cut it in half at the crease in the middle.
Flip all the squares over so that they face picture-side up. Now you can figure out in which order you want them in.
Once you know where you want which pictures to be, tape the backs of the pictures together into 4 rows of 3 (you could also do 3 rows of 4 if you want a wider, shorter poster). Once you're done, tape the rows together as shown in the picture.
Flip over and you're done!