The perspective on that pic ain't that bad; if anything, the Patchouli figure should be altered to be as if seen from above, since the rest of the pic seems to be as if seen by a security camera on the ceiling.
In the case of pictures like what you showed there, I'd draw what the character is sitting on first, then the character, and then the rest of the scenery. Before all that, though, I decide the angle of the whole pic, so I get an idea of how I change the contours of all the content to match said perspective.
Oh, and drawing the edges of rectangular things? I just experiment by changing th angles of them over and over, asking myself just what angle I'm looking at it per try. My prototype of one of my pics had the sofa looking as if from above, when I just needed to angle only half the edges to make it seem as if from the 3/4 view.
I hope none of that is confusing; brevity isn't one of my strong suits :ohdear: