The following is where the third step of the 16 steps of breathing meditation is explained in the sutra itself:
“One is mindfully aware of various breaths suffusing the whole body, as one attends to the exhalation and inhalation of the breath. As one pervasively observes the various kinds of inhalation and exhalation inside the body, one becomes aware and comprehends what is happening throughout the body, up to and including one’s toes and pores—[awareness] pervades as if water seeps into sands. In the same way, with [each] out-breath, awareness and understanding pervade—from the toes to the hairs, permeating all the pores—as if water seeps into sands. Just like a sack that is completely filled from its bottom to its opening, so too should one experience the body being saturated this way with [each] in-breath coming in from mouth [and/or] nose. One should perceive that throughout the body, where ‘wind’ traverses, it is as if it traverses through the holes of a lotus root; it is as if it traverses through the eyes of a fish net.
Furthermore, one should not just perceive the breath as going in and out of one’s mouth [and/or] nose; one should also see that the breath comes in and out from all the pores and from the nine orifices of the body. For this reason, one should understand that the breath pervades throughout the body."