Honestly CSing on Ahri just takes some time and practice, learn the timing and damage amounts as well as the animation. Always keep in mind Ahri is weak pre-6 so you're not likely going to be getting kills then so try and focus on CSing, harass and keeping wards up. Pressuring with Ahri involves using her Q and catching people out when they make mistakes with your E and make sure you try and hit the Q at the tip of the skillshot so you get both parts of the damage off at once. I would however recommend being careful with charm in some match ups for safety such as Leblanc, Zed post 6 and Talon. You should also try and weave your auto attacks into your spell rotations as it will add quite a bit more damage to your harass and all-ins not to mention your W will prioritize whoever you're auto attacking at the time. Something else is you need to understand is how much damage you and your opponent can do at a given time to know when you can all in someone. Given the fact that Ahri is a Mage/Assassin she can be played in two ways, Poke mage and Assassin and its vital to know which of the two ways you should be playing at any given time and knowing her all-in damage and when to go in is one of the most important things to know about playing Ahri, keep in mind you can use one of your ults in place to add more damage without moving. Roam with Ahri when you see the opportunity to help your teammates out, post-6 she is a god at roaming to other lanes and helping them get ahead, when against a hard match up this is one good way of snowballing/snowballing another lane when you're not likely to do well in your own. Sorry if this was hard to read or seems like I was rambling I typed this in a hurry. If you have anymore questions just ask via PM or a reply. Oh, and I'm sure you've already seen it but here's a 10/10 Ahri Guide that I've used and continue to use.