Recently played a game in the Ruy Lopez opening against someone who hasn't played much chess lately. I've studied the opening theory for the Ruy Lopez some and know some of the lines. I don't think my oppnent knew the opening theory at all.
I thought the game was instructive because it highlights a few important ideas in the opening.
At this point I was convinced that Black was completely lost. The most important points of the position are that the knight on a5 and the bishop on b7 are terrible. They are both trapped with no easy way to get back into play and aren't doing anything useful. The bishop on f8 is also trapped, and although it can get into the game more easily after the queen moves it is blocking in the 2nd rook. In short black's pieces are very uncordinated and not threatening anything.
In contrast White only has a couple problems with plenty of threats. The main problem I was concerned with was how best to get my dark square bishop into the game along with my other knight. While Black's king looked vulnerable to me I also knew that if I took too long there was a chance the Black pieces would eventually sort themselves out. Fortunately in this instance Black helped to open the center and my pieces have lots of space to move.
Usually when you see opposite side castling it is time for pawn storms. I considered it, however this option seemed too slow to me and I was concerned that moving my queen side pawns would give away squares to the knight on a5, allowing it to get back into the game more easily. Instead I decided to try and pove that my ability to control the diagonals to his king was crushing.
So there are several main things to learn from Black's play:
1) Be aware of a move's goal. Ideally a move should be trying to accomplish more than one thing at a time. Don't make other moves later that are counter to this original goal.
2) If you aren't sure what to do, and all your pieces are not in play yet, you should etiher develop a piece or make a move which allows you to develop a piece. Although I'm convinced Black was already lost by about move 12, completing development would have made made it more difficult to win. Material was still fairly equal at that point.
3) When making moves be careful of trapping your pieces. Even though material might be equal, if 1 or 2 of your pieces are unable to participate in the game it is the same as if they'd been taken.