Sorry, don't have one so I'll try to explain it more.
If you lay down on your stomach in the middle of a room there is going to be one wall that your hands are pointing towards (assuming your arms are overhead) and one that your legs are pointing towards. Next try to reach your legs towards the wall (as if you are trying to make yourself taller/being stretched on a rack). This should straighten your legs and extend the hips (which will activate the glutes). Now try to lift the legs while maintaining this "tall" position.
Does that help?