It is important to note in freezing that:
1) Use wards to avoid enemy gank (due to the fact that your pushing, which means easier target).
2) To scare your enemy and make him farm under tower or you won't be able to farm.
Step 1: Be Aggressive Towards the Enemy
First of all, before you freeze you need to control you lane. You control your lane by making your enemy farm under tower. after you scared the enemy top lane you need to stop the farm from coming to his tower or at least make him loss many farm, so that you would have more farm.
Step 2: How to Freeze
the way you can freeze is by only last heating the minions, so they would focus on the minions and the enemy will loss most of the farm.