"How to learn any foreign language" was written by B. Farber longer than 40 years ago. In spite of the fact you can find some aspects that can seems obsolete, there are some current techniques and methods that I consider essential and universal in order to learn a foreign language.
We have to take into account that in the age when Mr. Farber decides to write the book, the Internet hasn't been created yet. Therefore, one of the most important and useful source that today we have in our domain so as to learn foreign languages, it didn't exist when Farber published his book. Despite that, there are a lot of techniques and methods that you can use in the current world.
Inside of this large assortment of methods and techniques, I want to remark one that I regard cheap and essential when it comes to enlarge our vocabulary. It's the merely creation of flashcards.
Flashcards are small pieces of paper where you write the foreign word in one side and its translation in your mother tongue in the other side. The advantage of this tool is its portability and fast creation. You only need a sheet of paper to create around 9 or 10 flashcards at the same time. You can carry them anywhere and use them at anytime.
If you want to learn more about Flashcards, you can read my blog about how I create them to improve our vocabulary enlargement.