what is the best why to study an opening

I would like to give some of my advices,

• Study what your opponents are playing, rather than studying cutting edge theory. You'll find yourself in situations when your opponent doesn't play the most challenging moves and still taking away the victory, and that's not pleasant. For example, The Sicilian Bowlder attack, and Mc donald attack these variations are not challenging against the Sicilian but are very annoying and that's what we face most commonly at this level. You gotta know what to do against less challenging replies. Otherwise they may take away your interest out of the opening. Sure once you get to the level when your opponents start playing cutting edge theory you can study that. 

How to do that?

Well there are some Databases you can find on various websites, and you can have access to the games and what is played against less challenging moves. Make sure to study the whole game and try to understand what's going on in the Position.

• When trying to decide on your opening moves you should also consider going to the less explored paths, ie moves that are not usually played in that position, that way the position you will get from an opening can be very interesting. This could be difficult and time consuming but it is a trick I think works very well. For example

• And this is a very common advice but I believe in it that's why I am writing it. Study games in your opening variation.

1. Prepare some variations with chessbase or paper books or whatever.
2. Play games with decent time control, where u able to think about some variations, and can remember your preparation.
3. after the game check where you was out of book.
4. compare position where you out of book with your repertoire variations.
5. No variations in your book? extend it with your own analysis with stockfish / opening explorer/ elsewhere. Spend some time on it, u can memorize it very well when deeply analyze new position.
6. Variation exist in your book? check variation and ideas behind it, compare with what you play in the game. memorize.

Memorizing to deeply not required. The most important is to know ideas and some forced lines, if exists. It is ok if u arrive in the same position again and not able to remember the line. Happens with me dozens amount of times especially in Grunfeld Defence. But anyway you should spend time again to study this line.

That is my approach as 2300-2400~ player.
Earlier I'm using chessbase and pgn databases from different books. Now I build my own web app for studying variations

It's funny - you need to learn the idea and moves with Hanging Pawns, but then practice, and see what people play against you in real life. example - I tried some things against e4, chose French, now... main lines for White are 3.Nc3 [regular], 3.Nd2 [Tarrasch]. But in practice, people play the Advanced - 3.e5. or the Exchange French [3.exd5, exd5 - very g_y], or 'something else' - usually 3.Nf3. so with all due respect to theory... theory also say 'bishop is stuck', in practice I see that the king is confused where to go, but bishop is okay... so...

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and, of horse, you need to know some sidelines - so you won't be bamboozled by cheap tricks.

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I remember learning about Italian Game with Hanging Pawns, then playing against Chessmaster 2100 [it has 5 second per move to think]. - I CRUSHED it - checkmate. - I know the idea, prog is guessing moves... yeah, ideas matter.

Do not worry about openings.
Just play. If you want look it up after the game. It will stick more.

Focus initially on fundamentals, principles, themes and plans.  You can fill in specifics later, as you continue to play and become familiar with the opening.  Minimize memorization, restricting it to the first few moves of lines that you feel are absolutely essential to know for the specific variation you intend to play.   

Introduction to The Italian Game & Evans Gambit...

https://www.chess.com/blog/RussBell/introduction-to-the-italian-game

https://www.chess.com/blog/RussBell