One of Nia's nine steps, Lateral Travel, is a way to move your body through the space from side to side. There are two versions, one where the feet simply step together and step apart as the image above captures and a second version called "ninja style" where one foot steps or crosses behind the other.
In the classic lateral travel, you simply step your feet together and apart, shifting your body weight as you place your feet. Look in the direction that you intend to step, think of stepping evenly as you open/close.
In the second, "ninja style", mindfully step one ball of the foot behind the other, allowing for space between front and back foot, and look to side you are traveling to direct your mind. Keep your hips and knees relaxed, decelerate down and push up off the back foot for stability.
Tip: Begin with small steps, keeping your feet under hips. Increase range of motion and depth of your "sink" as you practice. Turn on some music, step 4 to the right, 4 to the left, 2 to the right, 2 to the left.... add variety and speed!