There are many factors too keep in mind, some already mentioned, some not:
1) Study on a 3-D board, not on a computer screen!
2) DO NOT MEMORIZE!
3) For each move, ask yourself what it is that player is trying to achieve. For example, after 1.e4, why would Black play 1...e6? Answer is to be able to play ...d5 to contest the White Pawn without having to recapture with the Queen if White decides to take it.
4) DO NOT MEMORIZE
5) By going through GM games, preferably annotated (i.e. Database is not the answer - a well-analyzed book with explanations by the author should be your approach - Also chesspublishing.com is not a bad idea), you need to understand the ideas for each side.
6) DO NOT MEMORIZE
7) Test yourself periodically. For example, after 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3, why is 5...Qb6 or 5...Nge7 or 5...Nh6 stronger than 5...Bd7? Because the latter does nothing to pressure d4, which is what Black usually does in most lines of the French (Exchange Variation excluded).
8) DO NOT MEMORIZE
9) Play in over the board tournaments with slow time controls applying the openings you are trying to learn. You also want to make sure the openings mesh well together. For example, you do not want to play as Black 1...c5 and 2...Nc6 against 1.c4 and 2.Nf3 if you aren't willing to play an accelerated dragon, like if you are a Caro-Kann player against 1.e4. After 1.c4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.e4 and you are in the Accelerated Dragon (1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 g6 5.c4 is the same position).
I hope you get the point. Anybody can memorize. Memorizing won't do you a lick of good. I can reel out a bunch of memorized moves. 1.d4 Nf6 2.c4 g6 3.Nc3 d5 4.cxd5 Nxd5 5.e4 Nxc3 6.bxc3 Bg7 7.Bc4 O-O 8.Ne2 c5 9.Be3 Nc6 10.O-O Bg4 11.f3 Na5 12.Bxf7+ Rxf7 13.fxg4 is the Seville Variation of the Grunfeld.
Ok, what happens if Black plays 8...Nc6 (without ...c5)?
What happens if Black delays castling and plays 7...c5?
What happens next after this line if followed all the way?
Answer? BEATS ME! That's because I do not "understand" the Grunfeld. I may know a couple of lines from memorization, but I don't get it, at all! If I tried to play the Grunfeld as Black, I'm screwed!
However, I do UNDERSTAND a number of other openings, such as the French, King's Indian, ...e6-Dutch Lines (Stonewall, Classical, Modern Variation, just not the Leningrad), and that is why I play them as Black.
In fact, just to give you an idea of the level of understanding I have in the French, it's not like I just figured out one line and went with it. As Black, against 1.e4 e6 2.d4 d5 3.Nc3, I could literally play any of the following:
Winawer with 7...Qc7 (Though now-a-days I don't play this line as Black due to 13.Rb1, which is what I play as White in Correspondence Chess - I play 3.e5 over the board).
Winawer with 7...O-O
Winawer with 7...Kf8
Winawer with 7...Nf5
Classical
McCutcheon
Rubinstein with 4...Nd7
And if you gave me a day, I could also play the Fort Knox (Rubinstein with 4...Bd7) or Burn variations, but I have not, by choice, played those over the board at all.
Anybody that tells you to memorize doesn't know what they are talking about!
EDIT: The OP (not me) deleted his own post, so it now appears I'm the first poster. I vaguely remember that the OP was asking about how to study opening books.
I assume you're studying an opening book, filled with selected lines and explanations by the author. You would like to have a physical board and play out the main lines. It's desirable to play out all the variations as well if your schedule permits. Take as much time on interesting positions, or moves that are difficult to decipher, as you want. Compare your thoughts with the author's.
About the use of boards: Personally I prefer using a physical board, (and most players around me do.) My head hurts when I try to look at "papers full of lines" with an imaginary board in my mind. Digital boards (for example, the Stockfish app on your phone) are possible, although it can be difficult not to get distracted by social media. Anyway, if you have to use an electronic device, remember NOT to activate the engine when learning openings. The silicon mind is frequently a source of confusion rather than clarification, unless you're rated above 2000 or so.
The best way to memorize lines, as far as I know, is to play actual games. A chess player only remembers lines (s)he understands, and that understanding usually comes from concentrated thinking during a game.