Good to hear that you're getting better wth Carl! Not many have the patience to use him. Give yourself a pat on the back!
As MIT said, Carl has low health so you have to be careful when being pressured. Dealing with pressure is very character dependent, so getting out for each character is way too much for a single post.
In general, you want to block until you see an opening (of course). Once you can see that weakpoint, you can jump, Vivance, or punish the attacker.
Jumping, if you can, puts you at a safer postition to control Ada. You can use j.A as an defense attack to keep your opponent away or j.2C to punish if you feel lucky. You'd be surprised on how useful j.A is. One Carl player got me to use it after he kept beating my j.B and j.C. ^^'
Vivance is risky to use (especially in CS), but is useful based on the situation. It is best used on someone who jumps a lot. Like jumping, you can control Ada while you escape away. If you use it predictably though... well, I once had a Tager KO me by grabbing me out of Vivance (while blocking Rhas. of Memo. too). If successful, you'll probably be put into a neutral position though.
2A, 5A, and 5B are your best tools to punish with. It's dangerous, but if it hits, it's go time!
As for keeping someone away... Brio, 5C, and Allegretto are all good too, but sort of situational.
I've noticed though, as Carl, a strong defense is a good offense. Once you catch them, don't let them go! Or else you'll need to catch them again somehow.
I hope that helps. That's how I remember it in CT, anyway.. good luck!