Of course reading a book that explains the concepts of an opening is probably the best way to go.
I can recommend another useful, but somewhat slower process. Record all of your games and go over them afterwards. When your opponent plays a move you were not prepared for, look up the position in an online database and learn what other people usually play for your side when facing that move. Keep track of the right moves somewhere (I use Word files) so you know what to play next time. Eventually you will learn your openings quite thoroughly.
If I can't find a good book on an opening, and I frequently have that problem, I use a couple of other sites when researching openings.
One is chesslab.com. You can put in a position from an opening and search for games. The search is customizable - for example you can look at all the wins for Black from a certain position and ignore other games.
There are three things I like about this site. First, it shows you all of the games from that position, ignoring the move orders that were used to get there, so you see all the relevant games. Second, it opens the game score in a window where you can use copy and paste to create a separate Word file of key games. Third, you can see the won/draw/loss statistics for any position.
The other site I use is chessopeningsdatabase.com They have won/loss statistics on every individual move. I use the statistics to focus my search on the moves that get the best results.If you click on a move you can open a pgn database of games where that move was played and play through all of the relevant games.
This site has some drawbacks, which is why I use it only as a supplement to chesslab.com. They do not show all of the games that reached a position, only the games that followed the move order you specify. They do not show drawn games. Copying and pasting of games scores can be done, but the technique is not user-friendly. And the 1.d4 section of the database doesn't contain enough games to be really useful.
I'm just beginning to appreciate openings having spent several months solely focused on tactics. Yes, my rating increased but I'm still a beginner (<1200). I think I can now begin to see the importance of knowing openings so as to improve my position in the middle game.
I wondered how do more advanced players use the Book Openings and Game explorer tab to improve their knowledge of openings?
I have been only using a Ruy Lopez ---and only a few moves into the opening--- for all of my games. Which openings and defenses should I next learn?
Thanks, Frank