The next step after relaxing the body is taking your awareness to the breath. Do not try to control your breathing, just let it flow naturally and observe.
First take your awareness to the tip of your nose and observe the air going in and out. You may notice that the air going in is slightly cooler than the air coming out. You may also be able to hear a sound as the air goes in and out.
Also observe the movement of your chest or abdomen and how this coincides with inhalation and exhalation. Is there a rhythm to the movement? Is there a difference between inhalation and exhalation? How deep and fully do you breathe? How fast or slowly do you breathe? And, is your breathing smooth or choppy?
Don’t rush this process, but also don’t take too long. About 2-3 minutes is fine.