Seek input that you can understand

Comprehensible input. You've been there, done that.

When you needed to learn Dutch, you read ‘Ot en Sien’ and ‘Kruimeltje’. Later, when you needed to learn English, you read Superman comics, the Famous Five and Myths and Legends from Many Lands.

You got yourself comprehensible input in the language that you had to learn. 

You chose interesting material at your level.

It’s possible to expand the difficulty limit of the material if you have a way to making it understandable. (Cheating is allowed.)

If you already know the story, that’s good (unless you are disinclined to re-read books or watch movies again and again). But you could get around that by seeking more by the same author, or continue in that genre.

If your listening is at a level where it supplements your understanding, then listen to the audiobook at the same time as you follow along on paper or the screen.

And if you read on screen, you might use an application that gives you the meaning of words when you scroll over them.

  There are means and ways.

Consider a spaced repetition system

A spaced repetition system helps you to review items on a list in random order according to how familiar you are with them. If you know them well, they pop up less frequently. If you are having trouble with them, you encounter them more often.

Anki is possibly the most well-known SRS online. I've experimented with it.

Currently, I use a site called kanji koohii to review kanji for my Japanese.

Nevertheless, I'm not going to recommend spaced repetition systems unreservedly for three reasons.

First, they can put you under some stress.

Second, they encourage the review of isolated words (though to be fair your learning list need not consist of single words).

Third, they are non-selective as to the importance of the elements that you review (unless you are very careful in choosing which items to include).

 To me, it makes more sense to read a book in your target language, because  words and structures come up naturally as often as they are important.

Read a home run book

According to Stephen Krashen, the term 'home run book' was coined by Clifton Fadiman in 1947. It refers to the first book that a child manages to read through in his or her first language. But I believe that it is just as important for anyone learning another language.

That's because it's quite an accomplishment. It gives you a great boost in confidence to complete an entire book in another language. You know that you can do it.

The experience also gives you a great dollop of second language exposure. Read a set of book at the right level (I'm thinking of Mami reading Enid Blyton's Famous Five series, and then C.S. Lewis's Narnian Chronicles) and you are 'in'!

You want something at the right level; something you are interested in; and something that you are familiar with. Illustrations help (there's a good reason why children read picture books first).