Before you move one leg back, stand with your feet 6-8 inches apart, then slowly move one leg back 'til you can get the knee of your front leg to line up with your ankle. This way you have a wider "stance" than you would if you start with your feet together before moving one leg back. I don't know about everyone else, but I find this wider stance better for my knees.
Also, use lighter weights. In PH, I remember Cathe saying that lunges is about balance so you don't need heavy weights. Or for now, use no weight at all until your body learns how to balance. Hope this helps.
Pinky