How to learn a language?
It depends on who you are, the way that is best for you.
Berlitz said learn the words you already know in a language.
Some are the same as in your language, some you’ve picked up over the years.
I find watching movies is a good way to become familiar with a language.
I’ve found that often subtitles are wrong.
Singing songs is the best for me but I’ve used all methods.

Let’s look at my learning German.
I started with a record course that my Aunt Jewel gave me and my brother.
I don’t think I learned anything from listening to it.
Then I took German in 8th and 9th and10th grades.
The lady teacher in 8th and 9th grades lived two blocks from me and she was a native born German.
I learned a little. We had readings and tests and a language fair at a distant university( I recited a German poem I think)
We learned some songs that I remembered later. By 8th grade the school had a language lab that consisted of tape recorded lessons that we listened to through earphones. I used those too.
In 10th grade, the man instructor had learned German as a soldier in Germany.
His methods were different but I saw that the things he taught had been in the course work in the earlier grads but I’d not remembered
the information.

Then when I was 19, I was called as a missionary and assigned to the German Zone of the Language Training Mission at BYU in Provo,Utah.
I was there for two months.
They wanted everyone to “live the language” meaning to just speak it.
How? by learning two phrases: “How do you say this?” “Wie sagt man das?”
and “What is this?” Was ist dass?”
We attended morning and afternoon classes.
The teachers were returned missionaries proficient in the language.
Part of the instruction included standing and the instructor saying a phrase and then telling us to repeat it aloud after him.
That worked well for most of the students.
But one fellow who’d been there longer than me, a Brother McOmber, never got very excited when his progress was slow.
He was older than the rest of us. He’d been a welder by profession.
His brother had been here too. The brother was a straight A student in school
and on scholarship to the University before the mission.
He failed at this “repeat me” method and left in mental breakdown because he couldn’t learn that way.
And we had classes in the evenings with native speakers who taught us the culture and their particular dialect.

Then I lived for 22 months in Austria speaking mostly German as a missionary.
Much of the German I spoke was about religion.
By the end of the 22 months, I found it difficult to speak in English
and two years later failed an English proficiency exam at BYU but got a B+ in German.
I’d not spoken or read much in German in those two years.

Later while living in Cottage Grove, Oregon, I met a retired railroad man.
He was from Italy and of course spoke Italian.
At 17 he and his family moved to Brazil so he learned Portuguese.
When he moved to the US and started work for the railroads, he learned English and Spanish.
The company would send all new Spanish speaking employees to him or English speakers wanting to learn Spanish.
He taught them those two questions (What is this? and How do you say this?) in the two languages
and the sent them to speak only the language to be learned for two weeks.
At the end of which time, they could get by in the new language.
Then it was just a matter of learning more vocabulary and phrases.

I’ve taken a French course and viewed courses in French, Japanese, German, Spanish and Italian on TV public broadcasting.
some are better than others. Now there are things online like courses, podcasts, translations, pronunciation, etc.
Some are better than others.
I’m learning other languages by learning to sing songs in those languages.
I ask for help with the pronunciation from native speakers who are also singers.
This is like the native speakers at the language training school.

I’ve looked at the Spanish and English ¬†and French courses some have.
The phrases would be well to learn but without a pronunciation guide of little value.
I know some Spanish but other words I’ve no glue how to say them.
A child copies what it hears but if it never hears, it can’t copy.
For adults learning another language, it must be heard or spelled phonetically.

I even need help sometimes with German words that are new to me as I’ve never heard those word spoken.
If I just fake a pronunciation, then I sound like a Japanese friend told me, I’d be speaking Niglish(a derisive term for Americans speaking Japanese like Americans) when with the heard voice most of the time a person can get their tongue around the word and sound like a native.
As a radio announcer in English, there were times when people corrected my pronunciation and they were right, I’d just not heard the words said right and hadn’t practiced saying the words the right way.
Japanese is easy if spelled phonetically.
Chinese or any other languages that has say squares, tells no one what is being said or how it should be said. It’s just squares to a non-speaker.

How to learn a language?

Well that’s different for each person but there are many ways to learn a language.

Find what works for you and start.

And find a Native speaker who you can practice with.