Before you embark on reading Harry Potter, I should note that this method is NOT for absolute beginner's. Whilst it is a children's novel, Harry Potter is quite literary in style, and you'll need a solid basis of vocabulary and grammar so that you don't feel overwhelmed. My advice would be:

• Make sure you understand most of the language's grammar, at least enough to recognise structures and forms, even if you can't use them


• Have a base of at least 1000 words

How can you do this? If you're not already intermediate, skim a grammar guide, take very light notes, and do the exercises if you feel like it. The goal is not to be perfect, but to understand. Just try to get through it as quickly as possible. Here are some grammar resources I recommend from the languages I study:

• Spanish:
  
  
  
  • Textbook ($): Practice Makes Perfect - Complete Spanish Grammar
  
  
  • Website: studyspanish.com
  
  
  
  


• Russian:
  
  
  
  • Textbook ($): The New Penguin Russian Course
  
  
  • Youtube: Russian grammar
  
  
  
  


• French:
  
  
  
  • Textbook ($): Practice Makes Perfect - Complete French Grammar
  
  
  • Website: Tex's French Grammar
  
  
  • NOTE: I've only sparingly used the above resources. I've never formally studied French grammar, as it is a heritage language for me, sorry!
  
  
  
  


• Japanese: 
  
  
  
  • Youtube: Cure Dolly- I cannot recommend this channel enough. It is the only thing that made Japanese grammar make sense to me after so much struggle.
  
  
  • Website: Tae Kim's Guide to Learning Japanese
  
  
  
  


• Italian:
  
  
  
  • Textbook ($): Practice Makes Perfect - Complete Italian Grammar
  
  
  • Youtube: Learn Italian With Lucrezia; Grammar
  
  
  
  

Through grammar study, you'll pick up some words, but probably not enough. I use Anki, a computer flashcard software, to memorise vocabulary, which I will talk more about in the next chapter. To memorise the most common 1000 words, go to the Anki shared decks, and look up your language. You should find a deck that fits your needs. If not, you'll probably have to make your own with a frequency list found on Google. I recommend for these words to practice both Target Language to Native Language recognition, and Native Language to Target Language recall, just to make sure you know these base words very well. 

Of course, if you already think you have a pretty solid base in the language, you don't have to do any of this. If you think you have a better method of getting to B1 (lower intermediate), do that instead! The big point I want to drive home is that going into Harry Potter as a total beginner is not worth the pain. 

Once you've finished the above "foundational study", you're ready to move on to really reading the book.