I'm going to skip my usual really, really, long explanation. Long story short: TFA is wrong. Many people know how to learn a language. Many people learn several languages because after learning how to learn one, they realised it's not that hard.

Main problem people have: they have unrealistic expectations. Children don't have a magic ability to learn languages. They take 10-20 years to learn up to adult level proficiency. They take 3-5 years to learn fluency up to a basic level. If you learn as fast as a child, it will take you 5 years to be able to speak like a 5 year old.

Technique is important, but it is late and I don't want to write a massive post about it (again). Here is the short version. Study every day. Don't miss a day. If you study 3 times a week, you will plateau at a very low level. If you do less than that, you won't get past baby level. You will eventually have to "study" at least 1 hour a day, but you can start with any small amount you want (even 5 minutes is enough) -- but don't skip days. If you miss a day (or several), you can't make up for it. Don't miss days.

Only study language you understand. If you can not understand a sentence at least 95%, you will have troubles (the 95% number did not come out of my ass... it is important). Always aim for 100% comprehension. Whether you study grammar explicitly or not is up to you -- do it if you enjoy it, don't if you don't. However, do not practice constructing sentences from "first principles" using grammar. This will mean that you acquire non-idiomatic language. Only use grammar to check that your grammar is correct, not to produce sentences.

In order to learn sentences, it is enough to expose yourself to them (as long as you understand them). I found it faster to memorise exemplars. YMMV. If using flash cards (with or without SRS), always practice from your native language to your target language. Never the other way around. It is important.

You must also practice forming sounds. You can "mirror" recorded audio, but sometimes it is too fast. To combat this, get recordings of songs and learn them. Try to perfect them. Record yourself and check to see how close you are.

Always try to practice with native language sources. It can be a live person, TV, radio, or written language. Reading written language is usually best for the bulk of your learning. This is because you can easily get a lot of it and you can easily go through it again and again. When reading, make sure you understand 95% (there's that number again). If you don't, find something simpler, or find out what the content means and read it again. If the latter, repeat the section every few days until it seems like your native language.

Take every opportunity to speak the language. However, beware. Most people go through a "closed period". This is a period where you can understand a lot more than you can say. Often you can't say anything. This is normal. It means you are around a 2-3 year old native level of speaking. Congratulations! Don't let it bother. Keep going and you will definitely break through it. Most failures happen because people quit (duh). Most people quit at this stage.

There is a lot more, but that's probably enough to get anyone started. Many languages share a lot of grammar and vocabulary. If your native an target language are related, you can often "learn" a language (to a child level of ability) pretty quickly (say a year or two). If you are choosing a very different language, it will take you longer (3-5 years). No matter which you choose, you won't get adult level of fluency and proficiency in less that 5-10 years. Anybody telling you differently is selling you something that doesn't exist. Languages are huge -- you have to be in to for the long haul.