Start by performing a forward lunge. Allow your back knee to touch the mat, and bring the front knee to meet it. Bring the opposite foot forward into lunge position, and then the other, returning to standing. You should be alternating legs each time.
Some general tips for lunging: keep the front knee in line with the ankle, try not to let your knee jut in front of the ankle or too much to either side, and put your weight into the front heel to propel yourself back to standing.
