It's a bad idea to practice online if being good offline is what you're going for. Online is laggy to the point that it's basically a different game entirely. The timing of nearly everything is different. So if you're playing online a ton and then turn around and play offline with friends, your gameplay is going to be very, very off.
So, how are you going to practice if you have no one to play with offline when your friends aren't around? CPUs. I'm serious about that. It takes a ton of pressure off of you, since you can make all the mistakes in the world and there's nobody to judge you for it. You'll learn the offline timing for everything. You'll get a feel for how neutral goes with your character and vs this or that character. You can practice vs a specific character over and over, or do what I do and set the CPU to random so that you can get a feel for fighting every character in the game. It is, after all, also important to know what the other characters can do, even obscure ones not seen often in competitive play. Because you never know if one of your friends is going to pick one of those obscure characters. Duck Hunt? I've heard that character is seen very, very rarely online, which is another reason online isn't the best for practice since you'll only get matchup experience vs the popular 20 or so characters most of the time. But when my friend picked Duck Hunt, instead of having no idea what to do, I was able to go in confidently since the CPU taught me how their moves function, and proceeded to 3-stock my friend with Wolf. CPUs aren't perfect. They make suboptimal plays, read your inputs like a psychic sometimes, and do really dumb things other times. But they're still a very worthwhile training tool. Definitely don't start on level 9s, though. Start at level 6 and either lower their difficulty if it's too much, or work your way up.
Do also learn what your combos and kill confirms are, and practice them in training mode. Be aware that CPUs in training mode don't DI, so it's necessary to fight CPUs in actual matches to get a feel for how things play out realistically. Oh, that reminds me, fighting CPUs is also good for getting used to DI'ing yourself, and teching, recovering, edgeguarding, etc. Use your time fighting CPUs to practice all these. Look up guides for your characters on YouTube and that should help you learn how to play them. Practice the things the guides teach you. Watch tournament VODs of your characters to see how top players play them. You'd be surprised what your brain will pick up just by watching.
I've done all this, and I've lost two games vs friends out of over 50 played.
