Watching replays or spectating matches with good players is a very good way to learn any game as a first step.
Afterwards, take what you've learned to single player Smash against hard AI. It won't teach you how to play the game against a human, but I've found the AI competent enough to get the basics down, at least : the mechanics of your main (good moves and bad moves, knowing which leave you vulnerable and which are safe), punishing whiffs, gimping recoveries, recognizing patterns (as an AI, it does have patterns of course, but being completely unpredictable is hard even for a human. Learning to recognize patterns will be very helpful against the vast majority of other players, especially the majority of Little Macs infesting the online mode), etc. You are also not penalized for lag and do not need to take into account input delay (I have a 25 mbps connection and while I can always tech offline, I've found it's much harder online for me for whatever reason - I assume it to be lag). Also, since the only stage in For Glory is FD, which is not the case in tournaments, it will help you familiarize yourself with the other stages used.
It is a good idea when playing against the AI to set up some learning goal for yourself. For instance, set yourself a definite area you'd like to improve : "I will only attack to punish whiffs and misses and will not play offensively", for instance, to get better at taking advantage of opportunities, "I will only shield attacks and grab" to get better at this specific kind of punishing method, "I will only dodge by using spotdodging/airdodging" to work these reflexes, "I will only use a specific move chain/combo like dthrow > utilt > uair/fair until I can kill or it no longer works" are all examples. You don't need to win, the AI doesn't care about you horsing around and limiting yourself, just learn the mechanics! Vary the characters you play against so you are familiar with all of them and their attacks.
Meanwhile, keep watching replays while developing your knowledge. Once you know the game enough, start analyzing them with a critical eye : what went wrong for the loser? What mistakes did he make? How did the other player capitalize on them? What patterns were present, and if there were none, how did the players mix it up? Keep mental or pen and paper notes, the latter is useful to review.
The next step is then to either play online against random people or get some sort of mentor/tutor who will help you further your learning. If it's someone you know, try playing matches against him/her and with his/her knowledge, limiting yourself as you have done with the AI previously to target only a few facets of the game at a time, so opportunities to practice can be fed to you in a realistic way. It's nice to ask your mentor to play a specific strategy and learn to counter it, the same way one might learn a martial art by sparring games. In Judo, we often use a "game" where one dude attacks and one dude can only defend. We can also limit the kind of throw used. It's a way of learning that works.
Attend tournaments and play friendlies or cozy up to some people, or maybe try asking on the boards. I've found many online game boards have some kind of a voluntary tutorship program - that might be a good idea here. You could also do all of the above at the same time, whenever a specific step tickles your fancy, but I like to keep offline until I feel I am relatively decent. This way, I avoid frustration and don't get discouraged at the early stages. It's also a way to boost confidence so I don't give up once the going gets rough against humans, it's the same principle that scammers use on their victims, the "investment principle". If a person has invested so much money already, even if they catch on that it might be a fishy deal, they are likely to truck on because if they don't, they automatically lose; if they do, even if it's a snowball's chance in Hell that the offer is legit, they still might win out. Basically as applied : "I've done all of this work already, all that's left is learning what's germane to playing against humans and tune up on the fundamentals, so why should I stop now and turn all my hours of work into a clear waste of time?"
I have learned many games this way, and I'm just starting to learn this one, but I'm seeing improvements by the day. Like you, I have played the series a long time but always casually. It's a method that works for me. Good luck!
