Combo's are one of the toughest shots in pool. Its why your good player and even top pro's will avoid them if at all possible!
Thinking like this will get you into trouble, yes combo's can get real tough real fast with angle & separation of the 2 OB's yes we get that. To those that say "there can be easy combo's", yea I think most of us get that to. I still have problems with combo's and don't think that is ever going to go away. I look at them as an opportunity not a drudgery.
I would rather have a combo (most of the time) than most 3rail or more kicks, wouldn't you? Combo's take more time to "see" than a single ball shot because there are 2 balls that you are coming into (most of the time, I am not talking about the frozen balls that you see instantly; I am assuming here there is an angle and there is some separation between the 2 OB's)
For me looking at the intended OB and then working backwards to the combo ball does not work, I also have not seen someone who is good at combo's use that method even though that is in all the books and is in a great many training videos. I (as of a couple of years ago) look at the 2 balls lined up in a straight line and then adjust from there, I still suck at combo's but that has certainly helped to get some semblance of accuracy.
As for caroms I use the stun shot about 80% of the time (maybe more), for short caroms I can now see "the line" pretty accurately. A little thing J.R. Calvert taught me many years ago was to drop your stick so that it is on the tangent line as close as you can to the carom ball you will see a reflection inside the ball that is a bow, this is a lot like when you drop your tip at the bottom of an OB and you see the reflection of the stick inside the ball to get the contact point. The carom ball will be "smiling"; shoot the CB to punch that smile right in the bottom lip. For long carom shots I still use this method today; it may not be conventional or scientific but it sure works for me.
As for how to practice and drill for these shots there isn't a lot of high grade, time tested drills and routines out there for this kind of stuff so you will pretty much have to invent your own. Cowboy is a great game and will teach you carom lines better than any other game IMHO. So many pool players don't see the "through" the 1st OB ever, they only see the "off the OB" possibility, so even though I use stun 80% of the time and "through" the 1st OB even less I still know it's there, and something to get you in touch with that kind of thing is always helpful in lots of different positional situations not just carom shots per say.
3 cushion billiards is only moderately helpful, but if you play it with pool balls it is more helpful. The drawback with using pool balls is that although you are more familiar with the size and weight and what you can do with them is that they make the game harder because they are smaller; so those "near misses" that you get with them is more and you will realize that if you had been using billiard balls that you would have made the hit quite often.
Another helpful game I like to play is what I call "kisses, caroms, combo's, banks & kicks", when you are playing a significantly inferior opponent you just don't shoot anything straight in, you will lose a lot in the beginning but it will really help you "special shot" play, I can play this because I no longer have any ego to bruise because after this long and not being a great player by now has made my ego just one big bruise; good luck.