Here is a demo vid that show good and bad technique and and an overhead variation.
http://www.youtube.com/watch?v=4qoYbW34tFU
Key point knee is never infront of the toe
                push up through the heel of your front foot.
Start with lunges then move to walking lunges.
Do you know why your ankle collapses?
Do you have flat feet?
What is your ankle rom like?
Pro Episode # 44: The Last 25% of Your Ankle Restriction (Snatches and Pistols here we come!)
Ankle Positional Fault Fix: Jill Miller Style
Free Your Heel, Free Your Mind
Episode 07: Bro, Your Navicular Bone Dropped

Are your hips tight?