I recommend reading those tutorials from Cyangmou that Ryumaru mentioned, they explain how perspective comes into play quite well. I think if that stuff clicks for you, you'll have a much easier time figuring out how to draw octagons (any any other shape) in a way that fits with the rest of your game.
Part 1,
part 2,
part 3. They're not that long a read.
A lot of it
is down to what looks good, but understanding the perspective/projection you're using makes it a lot easier to arrive at a result that looks good without a lot of trial and error. While there might not be "set formulas", there are broad rules you can follow to figure out, for example, where the gable on a house roof should be placed to make the roof look gabled rather than like a lean-to, or to figure out the size and shape of the top of a barrel or crate so that those props look like they exist in the same space.
Same with the boxes above the crates example, the left one is a box lying down, middle is a cube and the right one is a box standing up.
This bit perplexes me. A cubic box is a cube no matter which side it's resting on, because all the sides are the same size. The top box is a box seen in isometric or 3/4 view, the bottom box (a square) is a box seen from directly above, or from the side (sidescroller view). If I encountered both of these in a 3/4 game, the square would look like a piece of cardboard or a flag or something, because it has no depth - if it had depth, we'd see it.