I have been doing cake for about 2 years, so still starting out, with fondant and I still have trouble getting my square cakes to look good. I have seen people gather the corners of the square cakes and cut the fondant, but how do you smooth it out so you can see any seams or patch jobs... make sense? How do you do it?
I like to roll my fondant a little thicker for square cakes. Don't roll it out too big or it will be harder to smooth down.
I chill my cakes for 40 minutes or more so they're really firm when I apply the fondant. This gives me the opportunity to be more assertive with my smoothing. I don't have to worry about smooshing anything.
I lay the fondant over the cake, then IMMEDIATELY smooth the top and top edges and corners. The quicker you get the top edge to adhere to the cake, the less likely it is to tear.
Smooth the fondant onto the corners first. Then, smooth the sides. If the fondant puckers, gently pull it away from the cake until the pucker/pleat opens, then smooth it down again. (This is why I like the fondant to be a little thicker.)
Here's how to gather the corners. Spritz a little water on your fondant before you lay it on the cake. Lay the fondant on the cake, water side down. Smooth down the sides. Press the corners together as close to the corners as possible. The water is what helps the fondant stick together.
Use a small pair of scissors or a sharp Xacto knife to make a straight cut down the corner. Use your fingers to smooth out the seam.
As with any technique, you'll get better with practice.