When I decide to study an opening I tend to get a book or watch a video on it. (something or someone who just doesn't tell you what the moves are but explains why those moves are made) later on after going through the book or videos I will go over some games in a database of players I respect. Then just play it over and over against something like chessmaster or rybka and see where and how they're getting an edge on me. I Don't aspect to draw or even win but if I can last equal for a decent amount of time into the middle game I am happy.
For an example:
Once I get some free time after the holidays. I have a couple grunfeld books I am planning to look through and later I will look carefully through games of players such as Kasparov, Fischer, Svidler, kasp and fischer because they're just amazing players but I heard svidler has some amazing results with the opening so I added him to my list also.
And that helps give me a deeper insight to what kind of attacks / defences are employed by both sides and gives me an idea of what kind of middle game themes you will run across sometimes will even give you an insight to endgames you maybe running into.
In short studying opening isn't memorizing moves but understanding what you do after the "book moves" are over with, in my opinion
When people say study the opening what do they mean exactly like whats next for sudying the opening after you have your opening repritore and know all the variations to the openings you play.