If you're just beginning to exercise, start with this essential plank exercises on Mondays, Wednesdays, and Fridays for 4 weeks.1) center plank 3 times in a row for 45 - 60 seconds each, 15 second rest in between
2) right side plank (demonstration video here starting at :42) - hold a side plank and lower your hip to the ground and then raise it up high,
3 sets of 10 reps, 15 second rest inbetween.
3) left side plank - repeat for the left side
Do those three exercises in a row during the week. Next week if you want some variety, you can do the planks in a round 3x - center plank once, right side plank, left side plank, then repeat the round 2 more times