how to memorize openings

Reading books and playing repeated games with that opening

"... Create a simple opening repertoire (understanding the moves are far more important than memorizing them). ..." - IM Jeremy Silman (December 27, 2018)
https://www.chess.com/article/view/little-things-that-help-your-game

Perhaps it would be a good idea to start with Discovering Chess Openings, a book about opening principles.
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf

Some of us think that it can be useful to use books like First Steps: 1 e4 e5 and First Steps: Queen's Gambit
https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf

https://chesscafe.com/book-reviews/first-steps-1-e4-e5-by-john-emms/
https://www.newinchess.com/media/wysiwyg/product_pdf/7652.pdf
as sources of games with explanations intended for those just starting to learn about an opening. Be sure to try to use the openings in games in between sessions of learning. Most of the time, one faces a position with no knowledge of a specific move indicated in a book. One has to accept that as part of chess, and think of opening knowledge as a sometimes helpful aid. After a game, it makes sense to try to look up the moves in a book and see if it has some indication of how one might have played better in the opening. Many opening books are part explanation and part reference material. The reference material is included in the text with the idea that one mostly skips it on a first reading, and looks at an individual item when it applies to a game that one has just played. Resist the temptation to try to turn a book into a mass memorization project. There are many important subjects that one should not neglect because of too much time on opening study.
https://www.chess.com/article/view/learning-an-opening-to-memorize-or-understand
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... Everyman Chess has started a new series aimed at those who want to understand the basics of an opening, i.e., the not-yet-so-strong players. ... I imagine [there] will be a long series based on the premise of bringing the basic ideas of an opening to the reader through plenty of introductory text, game annotations, hints, plans and much more. ..." - FM Carsten Hansen (2002)
https://web.archive.org/web/20140627055734/http://www.chesscafe.com/text/hansen38.pdf
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)
"... Review each of your games, identifying opening (and other) mistakes with the goal of not repeatedly making the same mistake. ... It is especially critical not to continually fall into opening traps – or even lines that result in difficult positions ..." - NM Dan Heisman (2007)
https://web.archive.org/web/20140627062646/http://www.chesscafe.com/text/heisman81.pdf

Memorizing openings is utterly useless.

You need to UNDERSTAND every move of the opening, not just memorize.

Let's take the advance French.  1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Qb6 6.a3

Can you answer the following?

1) What should White do if 5...Bd7 instead?

2) Why does the answer to number 1 not work well after 5...Qb6?

3) What are Black's 2 good options after 6.a3?

4) Why are other 5th moves besides 5...Qb6, 5...Nge7, 5...Nh6, 5...Bd7?! even worse? (For example, I faced 5...f5, and won, in January).

If you cannot answer these questions, your "memorizing" will be totally useless as your opponent is under no obligation to play the best moves.  You must know and understand what you are studying, not just memorize reams of lines.

Even if you memorize through move 33, if your opponent did the same, what do you do now?  If all you did was memorize, once again, you'd have no idea what to do after that!

If you have a lot of theory you need to memorize, chessable should be your friend. 

I do think there's a level at which knowing a lot of theory is important, or at least can be helpful. Given your ratings though I am not sure you are at that level yet. 

Around 2010, IM John Watson wrote, "... For players with very limited experience, ... the Sicilian Defence ... normally leaves you with little room to manoeuvre and is best left until your positional skills develop. ... I'm still not excited about my students playing the Sicilian Defence at [the stage where they have a moderate level of experience and some opening competence], because it almost always means playing with less space and development, and in some cases with exotic and not particularly instructive pawn-structures. ... if you're taking the Sicilian up at [say, 1700 Elo and above], you should put in a lot of serious study time, as well as commit to playing it for a few years. ..."

"... The KIA is a good system to play. But unless you understand the 'why' behind where the pieces and pawns go, its not really going to matter. ..." - SoupTime4 (~1 day ago)

https://www.chess.com/forum/view/chess-openings/kings-indian-attack-47

My fellow chess friend, L, has amazing memory. He's rated around 2100 and do 20-move-long opening preparation before any OTB game. L speaks in the fashion of opening experts and frequently refers to convincing analysis material stocked in their heads.

Despite having remembered everything, he always complains about "unfamiliar" positions in his games because the opponent "knows absolutely nothing" and plays the engine's 3rd or 4th choice at a certain point. Now that memory doesn't help any more, he makes horrible positional mistakes and spoils all the advantage he has got up to this point.

And L believes he has done nothing wrong; it's the ignorance of the other player, after all, that drags him into in-analyzed waters. When I tried to explain that positional understanding will help avoid those mistakes, he just shrugged and reiterated that he has memorized all the best moves.

Don't. Don't try memorizing any opening. Understand principles, be familiar with more common openings, but do not try to memorize any opening.

Save memorization when you're 1500.. 

The answers to these questions - and more - can be found here:

http://charlottechesscenter.blogspot.com/2017/08/opening-preparation-french-defense.html

Good to see you are keeping IMBacon's much missed sarcasm with the name.  All we need is the Ghost and things will be back to normal.

Now I guess the next question is:

Are you aware that the author of Post #4 (and 16!) and the author of that page that you put the link to are the same person?  :-)

Not sure about Matt, but I knew that. As a relative newcomer to playing the French, I have found your articles to be invaluable. Besides the one Matt mentioned, you have written many French Connection articles addressing specific variations. Those articles are 1/3 of my attempt to understand the French, alongside McDonald's "Mastering the French" and Uhlmann's "Winning with the French Defense".

And yet, the funny thing is, amongst my favorite French games is one I published before The French Connection even began!

http://charlottechesscenter.blogspot.com/2017/07/game-analysis-charlottesville-open.html?m=1

Round 3 specifically!

Round 5 was also a French, but ended with a disappointing blunder my me, converting a win into merely a draw.

Other favorites of mines are probably Volumes 12 and I think it is 23 and 24 (might be 22 and 23).  The two in Michigan where I was White in the first and Black in the second.

The 2 in Asheville this year in January (published in February if memory serves me right) were not bad either.  The one as Black where White fell for the 10.Nc3?? trap and the one as White that I even concluded that it was hard to look at being an advocate of the French seeing Black go down the way he did.

Most of my better games in the French have come during those summer road trips I have taken the last 3 years.  Hopefully things will open back up by late July or else this year may not happen.  I will include what I can, just the quality may be a tad lower if 2020 ends up being all rapid events on Tuesdays and Fridays.

Round 2 tonight cannot be published as an article.  The game was garbage.  I won, but it was total trash.

I had Black in the following game:  1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Qb6 5.a3 Nc6 6.b4? cxd4 7.cxd4 Qxd4 8.Qxd4 Nxd4 9.Bb2 Nc2+ 0-1 (yes, white literally resigned here!)

Yes I did - guess I'm a ThrillerFanFan.

Actually you referenced it in a post years ago and I had it bookmarked.

Actually, with the number of people that do play on here, yes!

Had this been over the board, no, unless I was facing some foolish kid.  About 90 percent of those I face over the board that is under 18 tends to force you to mate them, no matter how stupid their position looks.  King and White pawn on h3 versus King and 2 Queens, they play on!  (I would if online with no increment and opponent has just a couple of ticks on the clock - not over the board or correspondence).

Only study openings when you're 2800+

But if you really insist on memorizing, there's stuff on the web that let's you drill opening lines. On ChessBase for example. See link.

But like somebody else mentioned, memorizing wont do you much good. You also need to understand.