Two-parter...

When covering a square/rectangle cake with fondant, how do you handle the corners? There is extra fondant that either needs to be cut or stretched, but not sure what is the best way to do it.

Also, if I want very square edges, like on a shoe box, how would I get that? I thought of pinching the edges, but that leaves a rounded indent from my fingertips and not really a 90 degree edge.

What works best for me is to place the cake/board on top of a smaller stable object like a bowl or another box. Then when you lay the fondant on top of it you don't have to worry so much about the corners because they fall below the cake. Then gently pull the fondant away from the cake as you smooth it down. If you look in my photos I did a shirt cake covered in fondant.

As for the corners, I have never tried to get sharp corners with fondant, but I would suggest you use two fondant smoothers to get the corner.

That's a great idea. Much easier to trim at the bottom too.

I think I know what you mean on the corners; use two smoothers at the same time and work them next to one another to get an edge.

This cake won't be completed until next week, but I'll let you know how it goes. I'm just the retentive type and have to know how to do everything beforehand.

To cover a square or rectangle, you start with the corners, smoothing them straight down.
After doing all 4 corners, then flare out the sides and smooth those down.

Done properly, you won't have any excess fondant to be cut away. Trim off the extra length with a pizza cutter and you're done.

HTH
Rae