To me characters in foreground are always starting point for the background. They define sizes of objects in background, and also it's position (you don't want something to overlap, you don't want a clutter of background objects around the figure, etc). If I draw background with perspective, it is figures that define first perspective lines I draw, and from them come other lines.
Integrating foreground with background comes to that angles have to match, proportions have to match, and lightning has to match. Angles aren't easy to match but lots of that comes from treating both foreground and background as if they belong to the same space (ie share perspective lines and all). Proportions - well you pay attention to how sizes of objects relate to sizes in foreground, particularly heights. You don't want a cabinet as high as a character or something. Lightning is probably the easiest of all, you just decide where a few light sources are, mark them with a pencil and then shade or vary line thickness based on light sources both on background and foreground. It's also nice to let characters interact with the background.
I don't think that these things are very easy, even for a pro who knows ins and outs.
Amount of details, to me, comes to this: you step back from the page and observe what is looks like from afar. It should not be uniform, which means level of details should vary from panel to panel. It should not look empty, which means you should have a panel or two that are rich with detail. It should also have some relief from that, which means panels that have minimal or no background (because you've previously establishes sense of place, you don't need it in every panel). The hardest thing for many, I think, is to know when to stop. Many of us have fear of emptiness and end up cluttering the page.
Composition, I dunno, I think purely for exercise you could try to experiment with abstract layouts and then fit characters into shapes that you made. Like, make random shapes and circles, have them vary in shape, size and hue, then pick the ones you like the most and try to use them as guidelines, to fit characters and objects into those compositions... Make figures be in different sizes (ie, closer or further from the camera), make some of them in shade, others in light, make some of them standing, others sitting, others yet gesticulating in some way. And so on , play around, try as many things as you can come up with, alternatively pay attention to the composition when you're watching movies or photos in magazines, try to abstract those compositions (ie extract what are the basic elements of it) and see if you can find any inspiration in them.