For the square cake, just keep gently pulling on those corners with one hand and smoothing with the other. It will work! (Don't ask me how, I think fondant is magic.)
If by "extra fondant" you mean the fondant at the bottom, yes, you cut it off right at the base of the cake. You can use a pizza cutter or fondant cutting wheel for this.
Here's a link to ateco's lessons on covering cakes in fondant.
It doesn't show pictures of square cakes being covered but the man does mention that when you cover a cake with corners you always cover the corners first and go from there. And always, top to bottom. But go ahead and check it out and see if it helps ya hun.
http://www.atecousa.net/learn/satin_ice_3.shtml
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