That depends on the language. I learned German by itself. That one is super logical with pronunciation being very manageable. But it has a very dense vocabulary and a rather illogical list of genders that require memorization. What's good about it is that it follows it's root of Latin and Roman entomology, just older and more distant from English. I would say if you have a reasonably strong English vocabulary, there will be a lot of words that are easy to memorize. The funny thing about German is though that as loaded as it is with rules, the Germans themselves are just as likely to break from those grammatical rules as we are with English. When I first traveled to Germany and spoke German, I was told that my version of German was too formal. It works really well in academic circles and business speak, but I come off as a little too over-the-top otherwise. I spent all of this time memorizing genders of nouns and most of the Germans don't even get them right! On the inverse though, there are many Germans that take pride in trying to construct their sentences in as complicated a way as is possible.
Spanish and Italian can probably be learned together. With the Spanish that I learned in school, I realized one day that I kinda knew Italian. Again, I learned system plus syntax and I was almost there. There were some vocabulary differences but they made sense where and why they differed. Also both are much more casual and forgiving.
French is more unique, kind of like German. But, it's a hair closer to English. French is all about the rules though like German. They even have an officially sanctioned and subsidized government entity that determines what is and isn't part of the French language. I took on French because of the level of sophistication. It's a challenge. I'm not there yet on that one.
When I was in grade school, I had a teacher who knew a ton about Latin and would teach us our vocabulary terms by splitting the words into parts and explaining the root words. She was also crazy serious about diagramming sentences and grammar. That all really seemed to make sense with me. It helped me see English as a system and then understand those systems in other languages. That's a big part of all of it. When you can train your brain to recognize the different parts of the sentence (subject, predicate, preposition, time, location, etc..), then you can begin to decode it.
Learning about how to break codes and being good at puzzles of all kinds also helps.
