How to build an opening repertoire / learn an opening?

Hello. I'm currently trying to know my openings better and improve my opening repertoire, but I don't know how to go about doing that.

I play the Colle as White and I know that system pretty well. It's the black openings that I'm worried about. I've played e5 for most of my chess life against e4 but I must admit that I just move whatever move and use logic in the opening. However, I feel like that is only a disadvantage to me. So I've decided recently to study openings. 

I've decided to play Slav against d4 and probably e5 against e4 (or Sicilian or Caro-kann maybe). But I don't know how to learn those lines. I have NCO but that never helps much because 1) there's too many variations to choose from 2) I feel like it's just memorizing moves. I want to understand the position. 

So how should I go about learning an opening / building an opening repertoire? Should I just search chessgames.com or use Fritz's opening database or buy chess books on the opening and devour them? My rating is around 1750 at the moment. Thanks for any replies. 

1790 USCF is actually pretty high, so I'm guessing that you already know a fair bit about openings.  Books can help you find out the key ideas, and analyzing your games to see where you went wrong is also another important step.

GM Mihail Marin has written a couple of very good books on 1.e4 e5: Beating the Open Games and A Spanish Repertoire for Black.  A rarity among GMs, he claims to reveal all of his secrets, in the hopes that it will spur him to come up with new ideas after the books are published.  He also looks at everything critically instead of accepting old judgements without thinking.  John Emms is another good writer who put out Play the Open Games as Black, and GM Nigel Davies has Play 1.e4 e5! A Complete Repertoire for Black in the Open Games.  I don't know as much about the Slav, but GM Glenn Flear has a book called Starting Out The Slav and Semi-Slav.  IM David Vigorito's Play the Semi-Slav is chock full of pithy little insights.

Once you have read up on some of the positional and tactical themes in each opening, you might want to go through your games, and annotate them looking to see where you went wrong and comparing them to games from GM practice to see if you can come up with some new ideas.  You also might want to work out a repertoire of lines that have worked for you, and revise it regularly as you play more games.

I'd suggest watching IM Danny Rensch's videos on pawn structures, especially the caro-slav, since you already play the Slav and mention maybe playing the Caro-Kann. Understanding the pawn structures is vital to really understanding the openings you're playing.

Here's a suggested sequence for watching the various pawn structure and other videos.

http://www.chess.com/forum/view/general/chess-videos-my-sequence-for-watching

Hi, Collide.

My method:

1) Pick a favorite player who plays a somewhat narrow repertoire. Kramnik was perfect for me, he is my favorite player, and doesn't play many openings.

2) Find a database of all his games, split them into white and black. Then use either chessbase, or if you're good with it, chess position trainer, to create an opening book from those databases.

3) Establish a routine of playing/checking with the repertoire and add more lines to your memory as you play. Like, you play a game, and then you come back to your opening book software to see where you drifted off from Kramnik's repertoire. And next time, try to stick to it.

About understanding vs memorizing:

I think "understanding" an opening comes with playing with it, few words can explain opening ideas, so learning an opening requires memorization/playing experience. Only then you'll begin to notice patterns, what to avoid, what to aim for etc. This is my humble opinion.

Another interesting take on this subject:

http://blog.chess.com/AndreaCoda/creating-an-opening-repertoire---my-method

To build an opening repertoire, should I select a common opening for both sides, and include many short variations(usually 7-9 moves) with lots of commentary on the ideas, positional imbalances, and plans? That is my method of building an opening repertoire.