I found this on tumblr the other day, sounded like an easy good plan to me! I assume that you are familiar with the 'sit-up' and 'push-up' term, just in case you're not familiar with 'planking', here is how to:
You just stay still in that position for as long as its mentioned in the calendar. You can set a timer in your phone.
Start on February before its too late, summer is 5 months away ;)