Engage the knee rest up for the leg not in use and disengage the knee rest for the active leg, then move the calf padding onto the active legs side. Then position yourself by placing your curling leg's lower calf or thereabouts against the padding and place your other leg into the knee padding.
Start the curl by lifting your foot up towards your glutes as far as comfortable, then slowly lower the weight back down in a controlled manner to just before full extension. Keep your foot in a fixed position throughout the curl. Use the grips and brace your core to stabilise the upper body. Repeat for the desired amount of repetitions then adjust for the other leg.
Avoid the use of excessive momentum to swing the weight up or drop it down.