To do a plank, get into position as if you were going to do a push up, except that your elbows are on the ground. So your toes are pointed and on the ground, elbows are directly below your shoulders, forearms facing forward. Head is facing forward, in line with your spine. Your butt is at the level of your spine – no lower and no higher. Your legs and butt are tight, and your core is engaged. Stay in this position for the suggested amount of time.
Make this easier by putting your knees down on the ground and/or doing it for a shorter amount of time than suggested.
Make this harder by putting a weighted plate on your back.