Starting from a Plank position, lean forward as much as possible.
Keep the arms straight and the shoulders engaged ( Shoulder protraction and Shoulder depression ).
If using the ground, you can turn your hands to the sides to decrease the pressure on the wrists.
With this exercise, it is difficult to measure the intensity level, because a slight change in angle can make it much harder. You can either accept to not measure the intensity precisely and adjust it to do sets of 10 to 20 seconds of hold. Or you can do it in front of a wall, measure the distance from your hands to the wall (the further away, the harder is the exercise) and lean forward until your head almost touches the wall.