1. Making the Monolingual leap
It is my conviction that one of the most powerful tools in your learning arsenal is eventually making the monolingual transition. However, am I a master of Japanese that can read anything without any trouble and thus the correct person to ask? No. But I’ve noticed a marked improvement in my own reading that coincided with making the step to going fully monolingual (WaniKani excluded 
). So, while I may not be the best person to ask, I’ll be doing a short write up based on what I’ve read concerning making the monolingual transition. Please look at the entirety of this article with a healthy amount of doubt, and do make up your own mind! 
Going fully monolingual isn’t a necessity to studying Japanese, and may even increase your discomfort with the language for months on end, but the potential rewards as you have to learn to comprehend new words, context and the like in Japanese is very immense. It helps you to gain a deeper grasp of the language, and will probably be helpful in shortening the time to fluency as you are potentially increasing the amount of exposure to the language.
Another (claimed) benefit is that it helps you get out of the mindset of translating from Japanese to another language in your head, and truly start thinking in Japanese.
The first caveat to going monolingual is looking at time spent reading. If you find that using a monolingual dictionary causes you to read less than you do while using a bilingual dictionary due to it being more draining, by all means, continue using a bilingual one. There are still several potential steps you can take to try and incorporate more time with a monolingual dictionary, but as stated previously it’s not a hard requirement on your path to fluency.
The first potential step you can make is doing the first look-up in a monolingual dictionary, but if you don’t immediately get the meaning (though I believe the struggle in figuring it out is beneficial!) you can look it up in a bilingual dictionary.
The other potential avenue is trying and using a monolingual exclusively until you reach a point of mental fatigue where you can’t be bothered anymore and then switching to a bilingual one. This will somewhat increase your time spent fully in Japanese, though your reading speed (at least in the beginning) will probably drop by quite a lot.
I can’t claim to be an expert on this matter. I slowly got my feet wet in the beginning, only taking a dip here or there, getting my toes wet with some monolingual definitions and the like. With only the last few months going pretty hard on monolingual dictionaries. It’s only been a while since I made the full monolingual leap I’m describing here myself, so please remember to keep all this advice with a grain of salt. It’s based on some stuff I’ve read on various blogs more than personal experience. In general the claim is that the transition to full monolingual look-ups will see a slowdown for several months, but later on it is claimed that you will actually gain reading speed which will make up for the time loss.
I feel like a certain familiarity with grammar and vocabulary are a necessity when it comes to making the monolingual leap. Sure, you can try and figure things out from the get-go, but stumbling on even the simplest of words and having to look up 10 words in a single definition, that each have their own definition in which you need to look up 10 more words just seems like a fool’s errand to me personally. I feel like the same sort of advice that is given when first diving into native material applies here. Make sure you have a foundation of at least the 1,000 most common words and a firm grasp on basic grammar (N5 and N4 would be preferable as a minimum), but this isn’t a hard set of rules. The most important thing is that you feel at least somewhat comfortable reading Japanese. If each sentence of every definition leaves you scratching your head, making the monolingual transition would be the ideal way of sending yourself of to the loony bin. However, waiting until you understand all definitions immediately on first read would be too long in my personal opinion. The uncertainty, ambiguity and need to figure it out by thinking about what they could mean is one of the most beneficial aspects of it all in my opinion.
But in general I’d advice : Only make the leap once you are only somewhat uncomfortable with making the leap.
How to go about it? I’d advice picking up several monolingual dictionaries personally, as sometimes I won’t understand the explanation in one for a certain word, but then one of the other ones has an entry that makes perfect sense (or the word might just not be in the one you own, which is another problem entirely). And then, just look stuff up and see if it clicks! I do personally advocate for the monokakido dictionaries app that I mention quite a few times throughout this thread, and I suggest giving a quick read through the explanations of all the dictionaries in this post to make up your mind on which ones to get, but if you don’t want to bother with all that, here are the ones available in the monokakido app in the order that I’d personally recommend them :
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明鏡国語辞典 第三版
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三省堂国語辞典 第八版
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三省堂 新明解国語辞典 第八版
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大辞林４
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日本国語大辞典
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大辞泉
- You need a specific Japanese word when writing a text
While I’d say 90+% of your language learning needs can be covered by going monolingual, this isn’t always the case when compiling your own texts. Whether it’s just a short tweet or post on this forum, or an in-depth article of some sort, trying to find the exact word you want to use by trying and explaining it in Japanese terms and hoping Google has got your back is a dreadful way to go about it. Simply look the word up in an J-E-J or E-J dictionary and go on your merry way! (Though using a thesaurus is always a good option if you do want to stay in a monolingual environment!)
- It’s a highly specific term that you’d barely understand in your native language
Sometimes you will come across a word that has a very specific meaning / nuance. Sometimes you can roughly guesstimate this word when looking it up in the dictionary, but there will be times when even reading up on all the entries, looking up all the words in those entries, and diving even deeper still leaves you confused at the end of it all. This can especially be the case when looking at some more technical texts, I myself have encountered it on many occasions when venturing to the more technical pages on the Japanese Wikipedia for example. When reading certain types of texts I consider it fine to “cheat” and look up the English meaning just because the mental load of understanding the text itself already takes up all brainpower. You’ll generally find that it becomes easier eventually, and once it does, it’s a good rule to switch out the bilingual for monolingual again though!
Either way, the best way in my eyes is to take the leap, you can always go back if you find it’s not your cup of tea! If you encounter any problems or have any further questions, feel free to mention them here as one of the main reasons for this threads existence is to help others make the transition 😄