What would be the best way to learn Japanese by yourself?
1. Do not use only Romaji
(In case you don’t know, it’s just the pronunciation of a word in English letters, i.e. おはよう= ohayou). Learning to read Hiragana and Katakana proficiently will help you pronounce words correctly, not in the way that non-Japanese speakers do(such as exaggerating pronunciation)
2. Learn Kanji
Yes, Kanji is difficult, but start with the easy and basic ones first. repetition is the one of the best ways to get it drilled into your brain, as well as writing/reading a bunch of sentences with it. Learning kanji will make reading MUCH MORE EASIER! kanjis are used to represent single words, and by learning them you can look at a word and instantly recognise what it is just by looking at the kanji. E.g 学 (as in 学校 for “school”) you will be able to easily recognise that words such as 修学旅行 have something to do with school because of the kanji you notice.
If you don’t have much time, I highly recommend Mochimochi app, based on Spaced-repetition technique which reminds you to review Kanji at the right time. Moreover, Mochimochi has about 5500+ Japanese words and all Kanji you need for JLPT or communicating, each course is divided into small lessons helping you learn quickly and easily. Give it a try!
3. Speech practice
Talk in Japanese when you can, if you know others who are learning/are fluent, have a conversation with them so they can help you and correct your grammar, etc. Listen to how native speakers talk and try to imitate them as well; get used to their tone.
4. Learn about the culture
You don’t have to delve deep into the whole history of Japan, but knowing some of the basics of the country will improve your understanding of Japanese. (such as the traditional houses, famous festivals/holidays, etc.). For me, when I love a culture, I also love to learn the language.
These are only a few tips, but assuming that you are a beginner the basics should be enough for now. Good luck!