What's the most efficient way to learn a foreign language without immersion?

If it's Spanish you would like to learn (I took a look at your Q&A list), spend an hour on the Spanish sections of YA every day.
People there write as they talk. You will learn a lot of idioms and slang and suddenly be able to understand phrases that would have been mysterious to you before.
Reading comics is a funny but excellent way to learn foreign languages - for the same reasons.

If you know the basics of a new language then these exercises will help to learn quickly 1. Listening to radio 2. Watching movies with subtitles 3. Reading newspaper 4. Find the native speaker and start talking 4. Mobile apps will help to improve your language basics.
OR
Enroll for the course in any language school. All the best!

Study, read, write and listen / watch movies in the language you want to learn...

So much depends on your personal experience and attitude in learning languages in general that a definite answer is impossible. It also depends on which language you want to learn - some languages benefit from different approaches.

An excellent help is speaking with a native speaker of the language.

When you absorb, involve deeply, please be immersed in a book/thought/work/your English Business or Trade=BUZ English=English..Hence immersed in your English BUZ=English to learn English !?

I was among those dinosaurus who were studying the language by book )). In a real sense it is not difficult to learn language without immersion but still you should have something that will "remind" you of the foreing language you want to know. I remember myself talking to myself making up sentences or just translating whatver comes to my mind or whatever I hear in a street or on TV or radio (at the time I learned the foreing language there was no internter).
So, yes, you can study the language without immersion but you still have to "work out" to make your brains remember and seal the words and phrases.

via Youtube - you'll very quickly pick up the basics because you'll learn the language of choice from a native.
How about Greek? Not difficult when you realise there are some `150,000 Greek words in English'.

https://www.youtube.com/watch?v=HB0xU-2BLYQ

Start where a baby starts.I mean start with the basics.Home,family,food,drink,body parts.Never quit learning and it will take a long time.

by practicing it talking to sonneone in that language that you want to learn

Try with a book and copy the content. You will notice that you are acquiring the symbols and the correct sequence. After a month, please get a dictionary to realize the meaning for each sequence. You have learned a lot