I think I used this analogy in another post related to something different, but learning to CS is somewhat like learning to juggle. You can beg people over and over again to teach you how to do it, and they'll try, but you really just need to practice it yourself for several hours before you'll be able to get it down. That said, in both activities, I can tell you how to use those hours as efficiently as possible.
Also, I recommend that you do this even if you're a support player. Not only will you sometimes need to play off-roles that require that you know how to CS well, but also sometimes you'll be holding lane for your ADC and so you need to know how to get as many of those creeps as possible.
So pick your favorite ADC (or any champion, really) and go into a custom by yourself. Run your standard rune/mastery setup, and open the game with cloth/5. (Why? Because you don't want to be used to having more AD or more MS than you might in a real game.) Go to botlane (or the lane of your choice), and start lasthitting. Don't attack a single time unless it's going to be a lasthit. The wave will start pushing to tower, and you'll get good practice lasthitting against a large wave.
After you've pushed all the way to tower one time, freeze for as long as possible. In theory, you should be able to freeze indefinitely, and as you get more practice in, you will be able to do so. Use whatever skills you want to clear creeps, but remember, you're trying to freeze, so using a Caitlyn Q on the entire wave isn't helping you accomplish your goal.
If you lose your freeze, don't give up! It's also important to practice lasthitting when your wave is pushing out hard. (This is why I don't start with a freeze from the very beginning, which you could do by tanking the enemy creeps in the first wave.)
Your second goal is to not use any potions. You don't want to be tanking the minions at the end of a wave very much at all, and you should have to use at most 1-2 potions, but aim for not needing any.
Stay in the game as long as you want, I prefer to go until the wave that spawned at 10 minutes gets to lane. If you stay for this long, perfect CS is 114. You definitely need to practice more if you're not getting 90 CS every time you do this, and you should probably keep practicing until you're around 100 per "game."
Some things you might find helpful:
- Try to keep 1-3 more enemy minions than allied minions, no more.
- Don't let the enemy cannon creep outlast your cannon creep by more than 1-2 autoattacks. It will rip through your minions.
- Don't let the freeze push beyond the halfway mark, or your wave will start pushing out unless you have a massive surplus of enemy creeps, in which case you'll have a hard time tanking them between waves.
- If the wave is reset completely, tank the enemy creeps right in front of the enemy tower until your wave pushes forward. This will cause the next wave to push towards your side.
- To practice lasthitting under tower, just accumulate a large stack and let it push to your turret.
- How quickly does the enemy creep wave accumulate in a normal wave? What about when there's a cannon creep present?
- How much health do you lose by tanking 2 ranged creeps? What about 6? (If you open boots or longsword sometimes, you'll get a better feeling for this.)
- How quickly can a cannon creep rip through your creeps when your cannon creep is already dead?
- What aggression patterns do creeps follow?
Once you've gotten good at this, 1v1 a friend. You're trying to get as much CS as possible; he's just trying to harass you and doesn't care about CS. You'll lose every single match, but it'll help you learn how to CS under pressure.