This might not be what you're looking for, and might be too basic so I apologise in advance. But just in case:
The easiest way I've found to focus on the hip joint is to focus on the location of the coin (small inner) pocket of most jeans. That's the hip joint - whenever I focus on glutes, I end up using my back.
Flexing a joint - that's a bicep curl. Extending, by definition, is the opposite - straightening the arm. Applying that to the hips, if you put your thumbs on the aforementioned coin pockets, you should crush them when you flex the hips and that area should open up and be pushed forward on extension of the hips.
Regarding the lifts themselves, the folks who've already posted know more than me. However, check out Cressey/Robertson's "Get Your Butt In Gear" as it addresses hip extension somewhat more specifically.