Using your first color (for Jan 1), chain 150-200 stitches, depending on how wide you want your blanket to be. I ended up starting with 150, looking at it, and then increasing to 200.
Turn your work and single crochet with the color for Jan 2 into the chain.
Repeat this process for the entire year, taking breaks to add the month divider color (also a single crochet row).
Don't forget to weave in the ends as you go, otherwise it will be such a mess to deal with later!