To begin, I should state that my basic maxim in fighting is "hands against hands, feet against feet." This is only proverbial; meaning that the upper and lower gates are most easily defended with the limbs that are already there. Obviously you wouldn't want to block a high kick with a high kick.
Using the steps for Peng means warding off or deflecting an incoming attack by taking the attack off its original line. It may be upward, downward or sideways Peng, and may deflect a kick (or a potential kick), or may redirect the attacker's footwork to keep him off the line of his hand attack.
To express Peng in the feet and legs, there are several methods:
1. As the attacker advances, stepping in an arch into a forward (bow) stance applies Peng energy sideways against his lead leg. This will lead him off his line and into a vulnerable position. Right against right and left against left is "taming" his lead leg; right against left and left against right is "opening the lower gate." Timing is imperative: your forward step must not be so early as to show your intent, nor so late that he has already placed his weight on his lead leg, which will compromise your stance. It must be done while he is moving, just before his step is securely on the ground.
2. Reverse of 1, using the inside arch of the backward breaking step (as in Repulse Monkey). The combination of Cai and Tui in the hands and Peng in the step is especially effective.
3. Upward Peng can be seen in Golden Rooster. Usually seen as a knee strike, the upward motion of the knee can also be used to slide an incoming kick upwards and to the side. The empty lower half of th leg is already poised for a counter-kick, should the opportunity arise. It can also be seen in the half-step connecting Raise Hands and White Crane, where the right leg is raised and then replaced before turning.
4. Downward Peng is more difficult to see, but generally it appears in any movement where the leg is raised before stepping (Parry and Punch; Raise Hands; the retracting portion of kicks, especially the Lotus Kick or the connection of Right Heel Kick to Parry and Punch where the leg is not put down between the forms). Downward Peng is tricky, because it begins with the leg raised. It is used to suppress an attempted kick by leading the energy downward and to the side.
Turning steps also afford good opportunities to deflect the opponent's legs, and have the special feature of applying the warding off in conjunction with he spiralling energy of the whole body.
The range of combat (close, medium, long) will determine whether you use the foot, shin or thigh of your active step as the point of contact. Practise in all ranges.
PLEASE be very careful. Knee injuries are very painful and take a long time to heal. The "attacker" should remain loose, and allow himself to fall with the movement if pushed off balance.