How to memorize opening lines effectively? (without buying memberships)

I find that most of the online interactive chess opening courses are quite expensive(except free studies in Lichess)

Do not memorize openings.
Just play and analyze your losses.
You will remember.

@tpr said ^

Just play and analyze your losses.

That's a good idea. In most cases the engine move works best.

It should be 'easy' to make a plan for this:

Learn Ideas First
Identify key pawn structures
Understand plans & piece roles
Study thematic tactics
Use Spaced Repetition (Active Recall)
Use Lichess Studies
Practice and Review Loop
Play focused blitz or rapid games with a willing opponent who will play that opening
Analyze games with an engine
Review master games

Standard stuff, really. Using an engine won't really help with the 'why' which is the most important part of learning a new opening. Not knowing the 'why' will end up making a similar move in a different, similar position which most likely be a crap move.

"... 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)
"... If [your opening] book contains illustrative games, it is worth playing these over first ... 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)

There is a lot of information on recall from people studying things like math and languages. Taking notes, writing by hand on paper helps: so see/hear/ say plus think plus write is a combination that does help recall. My friends who are obsessed with learning new languages send a lot of time just writing words in notebooks again and again and...
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So I think making your own chess booklets by hand helps.
What I think hekps the most is playing-a lot-- and second to the most isplaying over annotated games moving the pieces on a physical board. Do that enough and somehow you start dropping the pieces on the right squares without thinking.
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At some point I mostly know & understand what I'm playing but there are a few positions where the right move is so anti-intuitive, its just hard to remember. I make a few flash cards for these few positions.

FWIW, that's what I do. Bill

@kindaspongey said ^

"... 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 gam...

I wish there were more books that were just collections of critical, annotated GM games in a single opening.

Lotuschess
Chesstempo Opening Trainer

(@)yangtuo

Never ever memorize openings, learn them.

@swimmerBill said in #7:

... I wish there were more books that were just collections of critical, annotated GM games in a single opening.

"... a group of 2200-2450 players. ... we were looking at the Sveshnikov 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 e5 6. Ndb5 d6 7.Bg5 a6 8. Na3 b5 9. Nd5 Be7 10. Bxf6 Bxf6 and I asked the players who played the Sveshnikov with Black 'What’s the main line here?'.
Every single one that I asked said 11. c3, which is completely absurd. Strong chessplayers know that 11. c4 is the main line now. ... 10 years ago an opening book on the Sveshnikov will probably just devote a minor footnote to the move that is currently the main line! The only way you can find out what openings are actually being played now and are in reality the main line, is to have an up to date database, and find a way to cull the games that are played by lower rated players and that are over a few years old. ..." - IM Greg Shahade (February 9, 2012)
https://www.uschess.org/index.php/February-/Greg-on-Chess-Opening-Books.html