Here's a good practice for anyone wanting to observe first hand how freezing/ slow pushing affects waves. I learn better hands on, so I did this a couple weeks ago and I now have a fairly good understanding of how I can interact with my creep waves.
Pick a champion you want to practise with. I recommend one with wave clear to make this excersize easier, but pick one that can't clear well if you want a good challenge. Start a custom game and leave it empty. You'll be playng this by yourself (no bots). Your objective? Take the enemy nexus. The "success" criteria:
-NEVER walk past the river
-NEVER lose one of your own turrets
If you're really nervous you can't meet that criteria, do your first practise worrying only about a single lane. Then try with only worrying about 2 lanes. Eventually make your goal to manage all 3 lanes with no towers lost. Right off the start, you can only manage one wave. You can't be in all three lanes for the first wave. So your priority is to start a slow push on your selected lane. Don't worry, your other lanes aren't going to lose a tower from unmanaged minions for quite a few early Waves. Once it's started, rotate to your second lane (reset that lane if you need to), and then repeat in your final lane. After you've got slow pushes started its about figuring out which lane needs your help and when. Which one is pushing back first? Which lane is snowballing backwards? Which lane needs a reset/freeze? Sometimes both top and bottom are snowballing backwards at the same time. try and figure out what you need to do to manage those waves so one can't snowball into your tower.
Here's a second challenge if you've got a friend willing to practise. Each of you on opposite teams, one of you in top lane, one in bottom. Same rule of never walking past the river, but here's the new catch. You also can't go past midlane. You're free to travel through the river on you half of them map. Top and bottom lanes get to go uncontested. You both get to slow push/freeze your lanes without opposition. This time it's a race to see who's minions get to the enemy nexus first. Mid lane is fair game. Each of you can roam to the mid lane to reverse the minion flow whenever you want. You can even fight and kill them here. Neither of you can go past the river or mid lane, though. So if they get away, leave them be. stay on your section of the map.
Don't expect either of these excersizes to be "fun." They are practise. But I guarantee if you can do them, you can learn a lot.