You start when the enemy has more minions than you do. Then you just last hit. Even though the enemy minions are dying at higher amounts of health, your minions will be taking more DPS. Here's a video of me freezing for almost four minutes. I could have done it indefinitely, but an enemy finally decided to show up. Something to take note of is something I did at the start; I brought the enemy minions away from my tower. If they're too close, enemy melee minions will start taking tower shots as well as putting the freeze line too close toward my side.
Also, I kill the minions at higher health because I need to stack Q. If you're doing it as a regular champion, you don't need the number difference between your minions and the enemy minions to be so big.
https://streamable.com/8lcvg