OK, I think that means that it’s time for you to start delving into the details. I’m not at the N1 level yet for Japanese, and I think it’ll take me a while (perhaps another 3 months of concentrated study, which is time I don’t have right now) before I get close to it. However, this is what I did to reach the C2 level for French, which is at least as high as the N1, if not higher. (C2 basically means fluency across all domains an adult native speaker would handle plus the ability to handle technical material in a specialised domain like science or the humanities. The reading-writing section of the C2 test included reading a 2000-word set of documents that needed to be used as the source of arguments for a 700-word essay. The listening-speaking section was listening to a radio interview followed by a summary, presentation and discussion with the examiners):
- Read news articles every day on topics that interested me, especially on technical topics (in my case, it was usually French politics or news on scientific research)
- Look up every single word I didn’t know in those articles without exception:
I had to learn everything. It didn’t matter if I forgot a few words afterwards, as long as I was very focused during the reading and look up sessions. I would read a sentence, stop when I found a word I didn’t know, look it up, and then continue reading.
- Use a monolingual dictionary as much as possible, and read dictionary entries in their entirety:
If necessary, I would look up words within the definitions that I didn’t know inside the monolingual dictionary as well so I could learn them all. I would only use an English-French dictionary if the definition hyperlink chain was becoming too long (say… more than 5-10 lookups within lookups). The reason I suggest reading all the definitions for each word is that you will get a much better understanding of how the word works in general, and you might even pick up useful expressions. Plus, this will allow your dictionary reading to become reading practice as well.
- Consume media for native speakers in the target language whenever possible as long as it interested me:
I often watched science programmes on ARTE, which is a French-German channel, and also listened to science broadcasts on Radio France Internationale. I also bought a quick guide to cellular biology for university students in French so I would be able to get familiar with the words used.
When the final preparation period came around, my main problem was listening comprehension, not reading comprehension, so I spent the month before the exam watching MythBusters in French.
In your case, you should probably focus on news articles for now while acquiring as much new vocabulary and grammatical knowledge from them as you can. If you want, you can also take a look at documentaries in Japanese or perhaps dig around in the high school catalogue of NHK for School, because that will probably expose you to complex topics in Japanese in a more interactive fashion that might be easier to follow. You should probably also try to see if you can find any patterns in the sorts of topics that come up in your practice passages so you can be sure that you’re choosing articles of a similar level when you’re doing your own reading.
That’s about all I can think of for now. All the very best!