Place a flat bench inside the Smith machine "cage" far enough back so that when your sit down and lower the bar onto your leg for a test, the bar will come down across your leg a few inches further away (towards your knee) from your mid-leg line. Once you set up, get in position by allowing the loaded bar to rest on your leg and your (one) foot (toes only) on a raised and stable platform. This is your starting position.
Raise your leg by pushing up with your toes as high as you can. Once at the top, hold for a fraction of a second and then lower the weight back down. Go down until your heel is a little below the platform.
Finally, repeat the entire movement for the desired or prescribed number of repetitions with one leg and then switch and do the same with the other leg.
Perform this exercise in a smooth and fluid motion
If the Smith Machine bar is cutting into your leg, place a folded (several times) yoga mat between the bar and your leg
Don't rush this movement; it's more effective when done slower
SPECIAL NOTES & LEVEL of DIFFICULTY VARIATIONS
To increase the level of difficulty, slow down the movement or increase the weight.