Step 1: Use the knowledge you already have
Did you know that adults actually learn faster than children?
Almost EVERYONE thinks that children have a big advantage in learning a language, but adults actually learn from hour to hour in less time (it's just usually longer, because we have jobs, hobbies, and a social life...).
As adults, we have the great advantage in learning a foreign language in that we intuitively understand (and correctly use on a daily basis) the grammatical structures of our native language.
Our only problem is that we probably haven't had anything to do with grammar since school, so our conscious understanding of English grammar is a bit rusty (what is a preposition anyway?)!
No wonder learning the grammatical concepts of German - not to mention the terminology - can quickly become intimidating, confusing, and overwhelming.
But I have good news: All you need is a quick refresher on English grammar to help you succeed in learning German.
Step 2: Create the right foundation.
While it may seem unglamorous, taking the time to understand how the German language works first (before focusing on vocabulary) will save you a lot of time, mistakes, and aggravation.
Trying to understand the structures of the German language (i.e. German sentence structure!) on your own can feel impossible. But a good guide will show you that there's actually a lot of sense and reason behind all the crazy little changes that probably have you confused.
If you learn the overarching principles and underlying patterns of how German works, you can then learn vocabulary and actually use it!
Step 3: Branching the smart way
Most learners of German make the mistake of starting to learn vocabulary at random, only to realize that they can't string that vocabulary together at all!
If you followed my first 2 steps and used your adult learning advantage AND built the right [grammar!] foundation, THEN you are ready to work hard on vocabulary (preferably thematically).
This 3rd step (which can take months or even years) is also when you shift focus and start working more on the classic skills of listening, reading, writing, and speaking (<- these are also areas that German learners often tackle too early).